[FEATURE] Let workspace users decide whom to send notifications

Workspaces are defined by having owners, members and default
notification recipients. These recipients, of course, need to be
a sub-set out of the owners/members crowd. Besides that, changing
the stage to "ready to publish execute" only considers owners and
falling back to "editing" stage only considers editors.

This feature request aims to allow editors/users to define whom
to notify, out of the owners/members crowd without limiting
these possibilities on the system level.

Resolves: #50224
Releases: 6.2
Change-Id: I39ded2ed02772040d498d5293d24c8bd6fb9ef46