model: move code from __init__.py to base.py

Having too much code, in particular too much imports, inside a package's
__init__.py is a recipe for circular imports, and considered bad practice in
Python [1]

Move out everything from kallithea/model/__init__.py to a new file
kallithea/model/base.py and adapt the existing imports.

[1] http://docs.python-guide.org/en/latest/writing/structure/#packages

model: refactor and simplify _get_instance

_get_instance is a BaseModel method, but never uses "self". Instead
it takes a class argument, which indicates that it's better suited as
a classmethod on said classes.

Also rename to something more descriptive, remove leading underscore
since it's not a private API, and refactor for readability.

model: drop BaseModel.cls

It no longer serves a purpose.

model: inline BaseModel.get_all

This is just needless indirection (and doesn't actually add any
abstraction on top of the database object), so inline all calls to it.

Don't touch PullRequestModel.get_all, though, since it is an entirely
unrelated method that just shadows the inherited get_all.

db: remove deprecated getAll method

Not sure why this wasn't done immediately when get_all was added. Maybe
search and replace was out of order at the time.

cleanup: make module self-naming consistent

The self naming is quite pointless ... but at least make it right.

cleanup: check for None object identity in cases where that is what the 'contract' says

Avoid applying bool() on complex objects - it might return something unexpected
such as the key (which might be 0 and thus be false). Checking for None is more
safe and faster.

Correct licensing information in individual files.

The top-level license file is now LICENSE.md.

Also, in various places where there should have been joint copyright holders
listed, a single copyright holder was listed. It does not appear easy to add
a link to a large list of copyright holders in these places, so it simply
refers to the fact that various authors hold copyright.

In future, if an easy method is discovered to link to a list from those
places, we should do so.

Finally, text is added to LICENSE.md to point to where the full list of
copyright holders is, and that Kallithea as a whole is GPLv3'd.

Second step in two-part process to rename directories.
This is the actual directory rename.