Provide an online list of available projects/languages

[julen]

Having an online index of the contents amaGama offers and the sync/retrieval date/time would be a big help for users.

Ideally this should be automated and exposed in a web front-end:

• Sync operations write/update the list of languages/projects/numbers in the server
• The updated data is read from somewhere in the server (be it a txt file, CSV, or the DB)
• Data is cached and served from there until a new sync operation invalidates the cache

[unho]

Commit https://github.com/translate/amagama/commit/f1172774 adds the ability to expose the existing source and target languages through amaGama API.