i18n: move list of translators to contributors file

Some .po files in kallithea/i18n contained lists of translators, present
from before usage of Weblate.

Move this information to contributor_data.py so all translation copyright
holders are given credits as CONTRIBUTORS and on the about page.

This commit is part of a series that normalizes the kallithea.po files by
removing all comments, in particular source code references.