Support opening an explain summary document before summary content is generated

[d13]

Open the virtual document immediately with metadata header and loading indicator, load summary when ready.

Verification steps

Explain a commit:

• see that the Markdown view opens immediately and updates its content when it's ready
• switch to another tab so this view stops being visible and switch back to the summary: you see "loading" screen for very short time that is updated to the full summary in a moment. Same happens when you drag tab to add a new tab-group and back.
• click "feedback". I don't know how to test it other than through the debugger, but debugging in aiFeedback.ts/sendFeedbackEvent() should show that all fields are substituted correctly: model, type, feature, usage, etc.
• Regenerate content. All actions above work in the same way after the regenerating

Reload window:

• You see an invitation to regenerate content. Regenerate.
• All actions above work in the same way:
  • survive layout modifications and tab switching,
  • feedback fields,
  • regenerate.