i18n: better stripping of header comment for new translations

Also strip any header occurrence of:

# FIRST AUTHOR <EMAIL@ADDRESS>, 2019.
#

while preserving stripping of:

#, fuzzy

and leading empty comment lines.

scripts/i18n: also normalize casing of UTF-8 in Content-Type

f626260a376c introduced invariant msgmerge casing. Do the same when normalizing
to ensure consistency also without msgmerge and to avoid unnecessary conflicts.

scripts/i18n: introduce --merge-pot-file to control normalization

There are actually *two* kinds of normalization:

- in main branches, where we just want the translations - not any trivially
derived information or temporary or unstructured data.
- in i18n branches, where we want the trivially derived information, and also
want to preserve any other information there might be in the .po files.

If no pot file is specifed, do it as on the main branches and strip everything
but actual translations. This mode will primarily be used when grafting or
rebasing changes from i18n branches.

When a pot file is specified, run GNU msgmerge with it on the po files. The pot
file should ideally be fully updated (as done by extract_messages). That will
establish a common baseline, leaving only the essential changes as needing merge.

If merging from default branches to 18n, it is better to skip .po and .pot in
first 'hg merge' pass, while resolving everything else. Then, with the
uncommitted merge, run 'extract_messages', and then merge the .po files using
--merge-pot-file kallithea/i18n/kallithea.pot .

(Actually, these two different modes could perhaps be auto detected ...)

scripts/i18n: add command 'normalized-diff'

Add a command 'normalized-diff' that takes two (po) files and diff normalized
copies of them.

scripts/i18n: add command 'normalize-po-files'

The translation files in the Kallithea repository contained references to
the location(s) of each string in the repository. This is useful to
translators, but is not needed for all other users.

The big problem with that information is that it changes very commonly as a
result of normal development in Kallithea, causing a lot of unimportant
delta in the Kallithea repository, thus causing unnecessary repository
growth.

In this commit, a basic version of the script is added, only containing the
code to normalize the translation files by removing generated and outdated
data.

This can be used to check or ensure internal consistency between code and
translations, by extracting and merging and then removing most of it again with
normalize-po-files:

./setup.py extract_messages

for po in kallithea/i18n/*/LC_MESSAGES/kallithea.po; do msgmerge --width=76 --backup=none --previous --update $po kallithea/i18n/kallithea.pot ; done

scripts/i18n normalize-po-files kallithea/i18n/kallithea.pot kallithea/i18n/*/LC_MESSAGES/kallithea.po

Includes contributions from Mads Kiilerich.

scripts/i18n: introduce new i18n maintenance script

The translation files in the Kallithea repository contained references to
the location(s) of each string in the repository. This is useful to
translators, but is not needed for all other users.

The big problem with that information is that it changes very commonly as a
result of normal development in Kallithea, causing a lot of unimportant
delta in the Kallithea repository, thus causing unnecessary repository
growth.

A script 'i18n' is added to help maintain the i18n files. Functionality will be
added later.