JetBrains: add preview metadata to popup

Created by: vdavid

Completes most of https://github.com/sourcegraph/sourcegraph/issues/36704 Fixes https://github.com/sourcegraph/sourcegraph/issues/36812

Added this part of the popup:

Also:

• moved some logic from the PreviewPanel (view) to the PreviewContent (model).
• fixed a bug in ThemeUtil::getHexString()
• made PreviewContent fields optional
• added a bunch of new fields in PreviewContent to allow for more match type-specific data
• fixed race condition around the UI element changes that was temporarily there for this branch

Test plan

• Check out this 2-min Loom demo
  • The two regressions: the broken (base64-encoded) preview and the disappeared preview for diff&commit results are fixed by the last, tiny commit!
• There are 33 (!) commits in this PR, so I decided to make this review easier by creating this 10-min run-through. Please watch this, and sorry for the whispering, just it's quite late, and I didn't want to wake anyone :D

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.