Add "limit" parameter to source and log queries
Add "limit" parameter to source and log queries

Summary:
This adds support for overriding `limit` in `/api/sources` and
`/api/sources/<id>/logs` queries, where `-1` means no limit.

Test Plan:
Tested against my changes to `starlog-client` (upcoming) that
uses this. Tested `/api/sources?limit=3`, saw 3 entries. Spun up web
client, verified that it still works.

Reviewers: brady

Reviewed By: brady

Differential Revision: D863

astranis/starlog,astranis/starlog,astranis/starlog,astranis/starlog