Facilitate group switching via personas

[andrewresearch]

• Existing authors have a default persona attached to their initial sign-in group
• Allow an author to create an additional persona which is attached to a different group
• Authors can name (and edit) their own personas - name appears in navbar along with switching/creating options
• Personas could use reflection tagging via system tags. e.g. groups become system level tags for specific personas (or default persona for sign-up group)