feat(transifex): Let apps keep languages they maintain in the repository

[luflow]

For #1006.

What this does

Commit 1 – keep languages. An app can add .tx/keep-languages with one language code per line:

de
de_DE

The sync then takes l10n/<language>.js|json for those languages from git again instead of overwriting or deleting them. All other languages sync as before. Apps without the file are not affected.

Commit 2 – lock those languages on Transifex. After tx push -s, lockLanguages.sh tags the source strings of the resource with locked_<language> (Preventing Translation Edits), so translators are not asked to work on a language that is never shipped.

• Only strings missing the tag are sent, existing tags are kept.
• A failure is logged and does not abort the sync.
• Tags are only added, never removed.

The commits are separate on purpose, so the second one can be dropped or changed.

Testing

• Commit 1: ran the changed branch loop under dash against throwaway repositories. The same test fails against master.
• Commit 2: not tested against Transifex. I have no access to the API with a token that may set tags.

Open questions

• Is the bot's token allowed to set tags? The help article says only admins and project maintainers can.
• Is locked_de_DE the right tag for a regional code? The article only shows locked_it.

If this is fine, I will send a paragraph for the developer manual to nextcloud/documentation.

[nickvergessen]

Don't really have the headspace at the moment to dive into this.