Minor command palette updates

What does this MR do and why?

Before we start rolling out the command palette to users, we need to make a couple of small tweaks

1. Start using different char to initiate project search - use ':' instead of '&' as the latter is kind of reserved for epics

2. Remove the search for recent issues initiated by # from the command palette scope. Using the # in command palette conflicts with the existing global search functionality to search for issue by its id. So meanwhile we remove the # from the command scope. But we are not removing the related code as we plan to have this feature but still need to figure out how to have both.

Screenshots or screen recordings

How to set up and validate locally

👣 Enabled the feature flag

rails c 
Feature.enable(:command_palette)

👣 Click on the Global Search icon in the new navigation (or / shortcut)

👣 Start @user, :project or use > to execute some default commands or jump to one of the menu items. In project scope you can also jump to project file when input starts with /.