Setup translation system

[LukasReschke]

Workflow should be:

• Initial import of the ownCloud translation
• Custom languages get translated there
• Translations are fetched from there and will be put into git automatically

Also add back #75 (comment)

See also https://help.nextcloud.com/t/translations-transifex/149

[MariusBluem]

Will Files Drop be translated in Nextcloud 9, because we have no Transifex yet 😕

[MorrisJobke]

Will Files Drop be translated in Nextcloud 9, because we have no Transifex yet 😕

@LukasReschke is working on this already ;)

[AndyScherzinger]

Sounds good to me, only thing that hasn't been mentioned but would be great to have is a bot that pushes the translation changes to their respective repositories.

[MariusBluem]

Just for my ToDo:

• Replace Transifex Link in CONTRIBUTING.md as soon as it is ready 😉

[MariusBluem]

⚠️ Another problem is, that files drop is currently not translated 😁

[MorrisJobke]

Regarding the rebranding: Could we try to do a search and replace for ownCloud to %s in the translations and hook in there the instance name of the theme? I could look into this soonish :) Then we could make use of all existing translations and import them afterwards