[FEATURE] Move Migrate Button into "Add Dashboard"

[sinkingpoint]

Description

This introduces a "Button Menu", a CRUD Button that has a drop down next to it for more options, not unlike Githubs Merge button etc. We then use that component to move the Migrate button from the header into an option below the Add Dashboard button on the home page.

This tidies up the UI a bit, but also sets the stage for a more generic "Import Dashboard" button that can be used to import dashboards from other sources.

Screenshots

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.
• Visual tests are stable and unlikely to be flaky.
  See Storybook
  and e2e docs for more details. Common issues
  include:
  • Is the data inconsistent? You need to mock API requests.
  • Does the time change? You need to use consistent time values or mock time utilities.
  • Does it have loading states? You need to wait for loading to complete.

[celian-garcia]

That's a good start!

Maybe we can have something a bit more extandable like this review changes view.

Something that when you click, it is opening a dropdown bigger allowing you to, directly from the dropdown, either

• Start a new -> Go to the new page
• Import from file/textarea
• Import from Grafana -> Grafana ID/textarea

It is a bit more work that does not fit in one PR only for sure. But shouldn't we brainstorm about that in a github discussion + figma ?

[sinkingpoint]

I'm working off of
https://www.figma.com/file/D9VznEcxMYrhyVwSb6fLuf/Perses-Design?type=design&node-id=708-10820&mode=design&t=2kaT4H74NmBauGny-0

which got shared a couple of meetings ago iirc

[Gladorme]

Nice!