Move and refactor Inserter component as reused in header tools

[aduth]

This pull request seeks to apply a few refactorings to the <Inserter /> component:

• Move from editor/inserter to editor/components/inserter
• Change entry point from components/inserter/button to components/inserter
  • Should enforce a general preference toward index.js as entry point of any component. If we want menu to be a separate independent component, we should move it to its own directory to reflect as such, but currently we don't have a need for this.
• Render <Inserter /> in Header tools

Testing Instructions:

Verify that an inserter button is shown both in the header and at the bottom of post content and can be toggled to display a menu overlay.

Future Tasks:

• Header inserter overlay position should open downward, not upward
• To the first point, we may want to consider extracting overlay/tooltip logic to a separate component (perhaps with logic detecting adapting to display in opposite direction when desired opening direction exceeds viewport bounds)

[youknowriad]

If we want menu to be a separate independent component, we should move it to its own directory to reflect as such, but currently we don't have a need for this.

Actually, this was the intention, since the inserter will be used without the button (typing /)

[youknowriad]

👍 LGTM