Amend + context menu in space header

[niquewoodhouse]

Your use case

In iA1 testing #19706 we've seen people become confused by the current options within the + button on a space.

Aim

When a space is selected, the + should only show options that directly influence the user's view of that space, and are consistent with other adding experiences in the UI

Tasks

• Remove start new chat [1]
• Amend explore rooms to direct users to explore 'space home', not open public directory (unless the space is Home or Rooms not in a space)
• Add 'add room' and 'add space' (as seen in right click on space avatar in sidebar (below screenshot)

[1] Would like to see this brought back in the future with a different UX, but there's ongoing discussions around this

Mockup