monitoring: grafana email notifier support

[bobheadxi]

Closes https://github.com/sourcegraph/sourcegraph/issues/11454

This PR adds:

• email support for notifiers
• automatic sync for SMTP settings from Sourcegraph site configuration

Implementation details:

• map relevant parts of all site config to a GrafanaChange
• changes now only have access to the new configuration (ie their goal is purely to apply incoming changes - this ties into my first question further down)
• add new wrapper around the grafana client, GrafanaController, to also allow stop/restart capabilities

Questions:

• failure behaviour (https://github.com/sourcegraph/sourcegraph/issues/11454#issuecomment-644576628 => https://github.com/sourcegraph/sourcegraph/pull/11554/files#diff-42ca82e5e966d29f64bb4a21c9e25983R194), currently:
  • nothing happens if problem is encountered by grafana-wrapper while performing update
  • if grafana exits with status code >2, grafana-wrapper exits (exit code 1 is emitted when Grafana is SIGINT'd so we ignore it)
• is EHLO == HELO? https://github.com/sourcegraph/sourcegraph/pull/11554/files#diff-229cb10705aabb2149674e4fd1a58bfeR30
• not sure what smtp.authentication does (don't seem to see an equivalent in the Grafana SMTP config) => I believe Grafana tries CRAM-MD5 PLAIN in that order

[codecov]

Codecov Report

Merging #11554 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #11554   +/-   ##
=======================================
  Coverage   47.44%   47.44%           
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  Files        1404     1404           
  Lines       79706    79706           
  Branches     6816     6816           
=======================================
  Hits        37817    37817           
  Misses      38310    38310           
  Partials     3579     3579           

Flag	Coverage Δ
#go	51.59% <0.00%> (ø)
#storybook	10.12% <0.00%> (ø)
#typescript	36.42% <0.00%> (ø)
#unit	47.04% <0.00%> (ø)

[unknwon]

Is this ready for review? Seems still have many TODOs left. If not ready yet, suggest convert to draft :)

[bobheadxi]

Ah good idea @unknwon sorry about that!

[bobheadxi]

Converting the draft for now due to change in plans: https://github.com/sourcegraph/sourcegraph/issues/11452#issuecomment-648652514 (will close when I have a new PR up)