New API endpoint to list issues

[La0]

This is the last development in the backend before porting code over to Treeherder.

Creation endpoints (revision, diff, issues) are OK now, but the bot also needs a way to retrieve issues in 3 stages:

• unresolved issues in a revision
• known issues (issues from that diff that are already present in DB - linked to another issue)
• closed issues (issues from previous diffs in the parent revision that are not linked to the targeted diff)

We can build a single endpoint that takes as URL parameters:

• Diff ID (it can be the PK as the bot requires these informations after creating its diff)
• mode unresolved|known|closed

The endpoint serializer returns each Issue ID & hash (that's it).

The Endpoint would be exposed as /diff/{diff_id}/issues/{mode}/

We'll then implement their usage in the bot in a followup