Dashboards: Add megawatt hour (MWh) unit

[zuchka]

What is this feature?

This PR adds the Megawatt Hour (MWh) unit to the dashboard editor. We currently have Wh and kWh:

Why do we need this feature?

community superuser and Grafana Champion @gpinkos is building some awesome dashboards for play.grafana.org that monitor the US energy grids. Having a MWh option would be ideal for formatting.

Who is this feature for?

grafana lovers, data-nerds, etc

Which issue(s) does this PR fix?:

N/A

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.

[mckn]

LGTM!

[morganchristiansson]

Very useful thanks!

Can I just add that Grafana on https://intermittent.energy/ already monitors US, Europe, Australia and Canary Islands electricity grids. @gpinkos may be interested.

[gpinkos]

Thanks for pointing that out @morganchristiansson . I checked out that site and it's awesome.

[morganchristiansson]

https://energygraph.info/ and https://skelectricity.info/ are two other grafana sites.
Feel free to reach out @gpinkos if you want to collaborate. We are on Twitter / X mostly.

Tho setup for play.grafaba.com should be easy enough to. eia.gov has all US data via api, tho with delay.
ElectricityMaps also has a repo on github which is a goldmine for grid data sources and discussions