read build ID from db in commonplace, command to store build ID to db (bug 1083185)


- After deployment to all the webheads, run a management command that reads the build_id.txt from each project, and stores the build IDs in the database (with each row being a different project, keyed by repo).
- When we're serving Commonplace's index.html, read the build ID from the database rather than reading directly from the build_id.txt file. Probably build in a fallback to the build_id.txt file if it's not in the DB.

Do we even need that fallback ? The only time where it won't be present will the for the first deploy, and we could have a default build_ids as part of the migration to mitigate that ?