Improve mentions navigation ux

[aonnikov]

When a user does not have access to a mentioned object, the mention link does not work and it is not clear why. We need to improve UX to clearly indicate what is going on and what user can do.

1. Suggest to join the space if it is public
2. Suggest to ask someone to add him if the space is private

_{Huly®: UBERF-8130}