[SpaceTime] In-place Markdown editor

[ThomThomson]

Summary

This PR adds a new markdown editor to Kibana:

In place

The new editor is completely in-place. This allows text content to be added to Dashboards without leaving the Dashboard context.

Conversion

This PR also adds an action that converts any legacy (Agg based) Markdown panel into the new in-place editable markdown.

Dashboard Links

Note

This element of the PR is under construction and if this is merged, this section should be saved for a follow-up.
Lastly, this PR adds the ability to specify Dashboard links in the new Markdown panel.

• Dashboard links in the markdown editor UI by clicking on the dashboard icon.
• When converting from a legacy markdown, any relative Dashboard link - starting with /app/dashboards will be automatically converted into this new style of link.
• New link styles are considered hard references. i.e. linked Dashboards will be exported alongside the Dashboard with the link.