abuse contact: footer link from [web] abuse_url, Abuse wiki page #323

Anyone can report abuse without an account.

• Config [web] abuse_url (empty by default; no link when unset). When set, the footer in layout.html links "Report abuse" next to Privacy. The footer must not hard-code gitbay.org: other instances run the same templates.
• For gitbay.org: a wiki page Abuse giving abuse@gitbay.org, what to include (URL of the repo, issue or comment; what is wrong), and what happens (the operator reads it, may disable the account or delete the content via admin user disable / admin repo delete). abuse_url points at it.
• Operator: create the abuse@gitbay.org mailbox or alias on Migadu.
• Admin wiki page: document abuse_url.

A report form (an abuse report command with an admin queue) is deferred until mail turns out not to be enough.