Improve performance of issues listing API endpoint

[La0]

Refs #2270

This is the implementation of my previous findings

• add index on Revision.head_changeset & Issue.path that are used in the various filters of the endpoint
• smaller serializer to only load & expose what's needed by the bot
• aggregate all filters into a dict to make Django ORM smarter and avoid joining on IssueLink multiple times

Using a prod dump on a local server (not tested on heroku), this gives a 25% to 30% speed boots (from ~1s down to 750ms).

Endpoint is still relatively slow, but to go further it would need a larger refactoring to discuss