Handle issues also, not just PRs

[danuker]

An issue should be handled correctly also:

https://github.com/chevah/salt-master/issues/1940#issuecomment-1092877637

Right now, only PRs are handled.

Log before deploying #43 :

File "/home/site/wwwroot/chevah/github_hooks_server/handler.py", line 204, in issue_comment
pull_url = event.content['issue']['pull_request']['html_url']
KeyError: 'pull_request'

After #43, and as per the last paragraph of #44, we will need to track "reviewers" of issues via the "Assignees" field again (as before that PR).