Design: User Space List - Find existing spaces to join, then request access

[bdellasc]

*final layout for the spaces table still TBD as of 6 Oct

Stakeholder feedback RE: existing spaces:

• the ui for finding existing spaces should be more separated from the user's current spaces. having all existing spaces under a toggle was confusing to people.
• private spaces should NEVER be discoverable via a search because of sensitive projects
• some felt browsing the entirety of the system wasn't the way they would want to find spaces - would want more of a search paradigm
• others felt browsing could help spark ideas on what kinds of spaces to look for, then filtering

Verification Conditions:

• refer to user flows/screens in the newest InVision doc
• design a UI flow that allows the user to
  • locate existing spaces that they'd like to participate in (via the +Add a Space link)
  • once located, the user can request access from the owner of a space
  • confirmation of request
  • confirmation of approval to join space, show new space in user's list
• Review with stakeholders