Changes to bounty/pull-request data model and logic
* save git sha of pull request's HEAD instead of possible merge commit
and save it for opened, merged and closed events
* only look for bounties being accepted in pull-request closed via
merge events (no need for compilicated logic with issue-closed event)
* pull_request now has issue_id column to make many sql queries
simpler
Frontend
* owner-bounties app-db structure changes
* bounties view with confirm pending payout UI
Other
* removed lots of unused legacy code
* bug fixes + refactoring
* added comments + TODOs
Teemu Patja