#6261416 Proof of Concept: Try adding a “My Themes” themes tab to the Theme Showcase

Changes proposed in this Pull Request

• Testing this concept as a partial solution to #54394 and #54251 — move “Uploaded themes” (ie. themes that have been used on this site previously) to their own tab for Atomic and Jetpack sites, so there’s no need for extra headings, and you don’t see duplicate themes side by side.
• I’d suggest, if we go this route, renaming the tab to something that more accurately describes what it does, since not all themes in this listing are “uploaded”. Maybe “My Themes” or something like that.

Before

After

Testing instructions

• Switch to this PR, navigate to /themes on a Jetpack or Atomic site
• You should see a tab for Uploaded themes that contains themes you’ve previously activated or uploaded
• Switch to a simple site; the Uploaded view and tab should not appear, and you should be switched to Recommended themes