Grafana: Adjust a few integration details

[amotl]

Hi again,

while working on GH-125, we've discovered a few spots on the Grafana subsystem which could be improved. Let us know if you have any objections.

With kind regards,
Andreas.

/cc @Tonkenfo, @wetterfrosch, @ClemensGruber

Datasource

• Use jsonData.httpMode="POST"
  Currently, no httpMode is set, so the data source will effectively use GET. This is very noisy in InfluxDB logs, and may even croak, especially with huge query statements.

Dashboard

• Panel: Skip non-numeric fields.
  Erledigt, see Grafana: Only permit numeric fields to be established on Graph panels #126.

• Editing: Populate "Notes" field
  When Kotori edits a dashboard, it should submit an edit message. A dedicated user to show up on "Updated by" would also be nice.

  

• Panel: Migrate to new Time series panel
  Kotori apparently still creates Graph panel (old) panel types. We should probably upgrade to the new Time series panel type?