Richer alerts; improved docs; easier testing

[jinnko]

You may not want to merge this, but thought I'd share just in case.

From the individual commit message:

• Alerts now:
  • link to the alertmanager URL and a dashboard if env vars are configured.
  • all labels other than explicitly formatted labels are included in the alert output
  • when annotation.url is set the link will be included in the alert
  • fixed extra spaces that broke the alert link
  • emit the alert severity instead of "firing" when available
• New logging module with configurable log level.
• Cleaner contianer

[jinnko]

@jelmer - yes - I intentionally make commits each with their own goal - so you're right they could be their own PRs. If you merge without squashing they will continue to be discrete changes in the history.

As they're all applied on my forked master branch - I guess the alternative would be to cherry pick each commit into it's own branch and raise multiple PRs - but the end result would be the same.

[jelmer]

@jinnko makes sense - FWIM I'm not a committer to matrix-alertmanager, just an interested contributor