Show "New post" or "Create space" button in the top menu

UX improvements on Subsocial:

1. Show "Create Space" in top menu if a user has no spaces yet.
2. Show "New Post" in top menu if a user has spaces.
   1. Redirect to "New Post" page if a user has only 1 space.
   2. Open "Space Selector" modal if a user has many spaces.