Logged in user can UNshare their boards with other users

[CPowell23]

As a logged in user
When I visit my board show page
I see a link to share the board

When I click the link to share
I see a popup titled "Invite people to join this board"
The popup includes a list of the users that I'm already sharing the board with
Next to each user, I see a link to "Remove" them from the board
When I click "Remove"
That user is removed from the board and I no longer see them on the list