2879: API server can find_or_create Jobs based on filters.

This will make it easier to support new criteria for find_or_create.
In particular, this will be the basis for searching based on Docker
image.
2879: API server can find_or_create Jobs based on Docker image.
2879: Use Job-specific filters in #index too.

This brings #index consistency with the #create method, and enables
clients to do the same kind of sophisticated searching.
2879: Support tag search in "in docker" Job filters.

The search uses the same `reponame:tagname` format used by
`docker tag`.
2879: Document new Job API filters.
2879: Job Docker filters respect reader tokens.
2879: Docker filters on Jobs accept an array of search terms.
Merge branch '2879-docker-image-job-reuse'

Closes #2879, #2924, #3030.