Suggestions endpoints for SDC image caption addition/translation
Adds endpoints for suggesting Commons files for SDC caption editing.
The suggestion algorithm is as follows:
- 50 random candidates files are requested from the MW API with basic imageinfo and globalusage data;
- Non-image files are filtered out, and captions are requested for the remaining candidates;
- Candidates are given a final validity check, and those that remain are returned.
Bug: T209997
Change-Id: I862bd382e4d93921a92467bd5a66435acd3ee53a