add 'rebuild' button to rebuild any build that has finished

There aren't nearly as many reasons to rebuild a commit as there are with some other build systems, but sometimes an http fetch fails and it's nice to be able to try it again.

This adds a /rebuild endpoint that re-queues the build job and redirects to the build.  I'm just learning go and familiarizing myself with drone, so there may be better ways to accomplish this.

References https://github.com/drone/drone/issues/126.