cleanup: get rid of most "import *"

Apply script generated with the following hack:
(
hg loc '*.py'|xargs pyflakes-2 | sed -rn "s/([^:]*):.*'(.*)' may be undefined, or defined from star imports.*/sed -ri 's,\\\\<\2\\\\>([^=]|$),XXXX.\2\\\\1,g' \1/gp" | sort -u
hg loc '*.py'|xargs pyflakes-2 | sed -rn "s/([^:]*):.* undefined name '(.*)'$/sed -ri 's,\\\\<\2\\\\>([^=]|$),XXXX.\2\\\\1,g' \1/gp" | sort -u
hg loc '*.py'|xargs pyflakes-2 | sed -rn "s/([^:]*):.*'(from .*)\.([^.]*) import \*' used.*/sed -ri 's,\\\\<XXXX\\\\.,\3.,g' \1/gp" | sort -u
hg loc '*.py'|xargs pyflakes-2 | sed -rn "s/([^:]*):.*'(from .*)\.([^.]*) import \*' used.*/sed -ri 's,\2\\\\.\3 .*,\2 import \3,g' \1/gp" | sort -u
) | grep -v kallithea/bin/kallithea_cli_ishell.py > fix2.sh

codingstyle: trivial whitespace fixes

Reported by flake8.

tests: add as little code as possible in __init__.py

kallithea/tests/__init__.py contained quite a lot of code, including the test
base class TestController. This in itself may be considered bad practice.

Specifically, this poses a problem when using pytest 3.0+, in which asserts
in some files are not automatically rewritten to give improved assert
output. That problem can be fixed by explicitly registering such files for
assertion rewriting, but that register call should be executed _before_ said
files are imported. I.e. if the register call is in
kallithea/tests/__init__.py, assert calls in __init__.py itself can not be
rewritten.

Since the TestController base class does effectively contain asserts, and we
do not want to execute the register call from somewhere outside the
kallithea/tests directory, we need to move the TestController class to
another file (kallithea/tests/base.py) so we can have a register call in
__init__.py before loading base.py.

While not strictly necessary to fix the mentioned pytest problem, we take
the opportunity to fully clean __init__.py and move everything to
the new kallithea/tests/base.py. While doing so, unnecessary imports are
removed, and imports are ordered alphabetically. Explicit imports of symbols
from modules that were already imported as a whole, are removed in favor of
fully qualifying the references (e.g. tempfile._RandomNameSequence).

pytest migration: rename TestControllerPytest back to TestController

The name TestControllerPytest was introduced to allow a temporary situation
where nose/unittest and pytest-based tests could coexist.
This situation is now over, so the base test class can be renamed again.

pytest migration: models: switch to standard assert statements

Use unittest2pytest to replace unittest-style assert statements (e.g.
assertEqual) with standard Python assert statements to benefit from pytest's
improved reporting on assert failures.

The conversion by unittest2pytest was correct except for:
- 'import pytest' is not automatically added when needed
- line wrapping in string formatting caused a syntax error in the
transformed code. Reported upstream at
https://github.com/pytest-dev/unittest2pytest/issues/3 .
- in assertRaises with a lambda, the lambda needs to be removed

pytest migration: model: convert all tests to TestControllerPytest

The model tests were based on BaseTestCase which does not exist currently
for pytest-style tests. Nevertheless, there seems to be no advantage of
directly subclassing BaseTestCase over subclassing TestControllerPytest.
Thus, keep things simple and use TestControllerPytest.

changeset status: fix common case where a reviewer did not yet review anything

When a reviewer did not yet make any review, its status for the changeset is
None, causing the code to crash. The unit tests also did not cover this case.

changeset status: set status to rejected if at least one reviewer rejected

Currently, a reject of a change by a reviewer does not affect the overall
status at all. A logical policy is to reject the change if at least one
reviewer rejects it.

Note: different repositories/organizations may require different policies
for the overall status. Currently, the policies for approve/reject are
fixed. A possible improvement is to provide several alternative
policies and allow selecting the policy per instance or per repo.

changeset_status: add unit tests for calculation of overall status

Add unit tests for the calculation of the review status of a changeset/pull
request. To allow this, some reorganization of the ChangesetStatusModel is
needed.