Users have been testing the threads sidebar on Nightly and Preview but
once we release the feature we expect many of these users to move back
to Stable. As it stands right now, they would lose the threads they'd
been working on. This PR adds a way to import these threads.

- [x] Add `agent: import threads from other channels` action to the
command palette.
- This can be always available for users that switch between channels
regularly.
- [x] Add tests for import behavior
- [x] Add a button that disappears after using it to make importing more
discoverable for when users first switch.

Release Notes:

- Added an action to the command palette that imports threads from other
Zed release channels into the current one.

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Co-authored-by: Danilo Leal <daniloleal09@gmail.com>