Review Mozilla 1905027: There should be a Keyboard shortcut for the AI chat sidebar

Bugzilla: https://bugzilla.mozilla.org/show_bug.cgi?id=1905027

Upstream wanted to use "Ctrl+Shift+L" to open the AI side bar. This was backed out in bugzilla bug 1927304.

The AI sidebar is already covered by #43562 (closed).

For this issue, we should:

• Check this wasn't re-enabled.
• Maybe have some means to ensure that this doesn't happen again for our existing shortcuts: "Ctrl+Shift+L" and "Ctrl+Shift+U" (any more?). Especially since mozilla localises their shortcuts, so it might conflict in another locale already.
  • Maybe ask mozilla to reserve these keys for fork-specific actions. E.g. they could add two or three <key> elements with cmd_extra1, cmd_extra2, etc, that are no-op upstream but we overwrite them in our fork.
  • Test that our <key> element takes priority over others. E.g. do we need to ensure it is placed before or after other elements?
  • Maybe lower down in <key> element's implementation we can explicitly give priority to our shortcut.