Increase locale column length from 7 to 10 characters (fixes #920)

zh_Hant_TW and other locales don't fit in 7 characters. 10 should be
enough.

Note that the migration doesn't work on SQLite due to limitations in
Alembic. In this case, simply stamp alembic to the newest revision (the
column character length definitions don't have any effect anyway):

    bin/alembic -c etc/adhocracy.ini stamp 1e296c7f5e8c
Merge pull request #924 from nidico/feature-920-increase-locale-column-length

Increase locale column length from 7 to 10 characters (fixes #920)