monitoring: grafana email notifier support

Created by: 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