[WIP] Add service to overwrite log content

See https://github.com/travis-ci/travis-ci/issues/890

TODO: The users's authorization should be checked
Check permissions and job state before overwriting logs
Clean up overwritten log

'reason' param may be empty, so avoid writing a dangling 'Reason: '.
Rename OverwriteLog service to RemoveLog
Tweak log removal message
Remove superfluous assignment

I believe `#log` is needed in specs
Merge pull request #343 from travis-ci/ha-feature-gh_ci_890

Adds ability to clear build log by authorized users