Added function to load translations from JS

For apps that support async translation loading, a new function
OC.L10N.load() can be used to asynchronously load the translations
for a given app.
Remove stray generateUrl
Better use of promise in OC.L10N.load()
Fix l10n promises
Merge pull request #12257 from owncloud/l10n-loadtranslationsfromjs

Added function to load translations from JS