Show a popup when user clicks on chain/unchain page action

[bitspook]

This will solve the problem that user can't have "free tabs", i.e tabs which
aren't tracked by Emacs at all.

The popup should have following elements:

1. A big button saying "Chain" or "Unchain"

   We are increasing UX friction, but offering more control.

2. A button saying 'Delete from Emacs?'

   This should appear only when the tab is unchained. We should perhaps replace
   the "Unchain" button's contents with an Insomnia-style prompt for a slick UI.

3. List of current tab's groups

   Dismiss-able badges to view and remove groups from current tab.

4. A UI to select or add new groups