Create user right for creating flow boards

This introduces a new user right, flow-create-board.  When a user has
this right and right they have the ability to convert a non-existent
wiki page into a flow board.  This is done from the API either via
edit-header or new-topic submodules.  By default this right is
assigned to the flow-bot group.

Bug: T76793
Change-Id: I0ad12ed81e818149fb6804eb3499ce5da3fc66de
(cherry picked from commit 210b31b0bc944dbf937f4db21d1768dc19b22b7f)