Allows admins to create mass invites and visitors to use them

[Manfred]

• Adds MassInvite for mass invites and MassInviteSignup to count users signups through the mass invite.
• Adds admin controllers and views to manage mass invites.
• Adds a user's origin so you can see if they were mass invites, invited, or were born from the dark dark void.
• Allow users to create an account through an active mass invite link.
• Move the user creation spam stuff into a concern in case we need it for other controllers that create accounts and maybe refactor the users controller¹.

Frontend Todos

• [x] Blank slate copy
• [x] Prettier route
• [x] After user creation, make it clear user needs to check email
• [x] Password fields should be on top of each other
• [x] Check out expired invite situation / copy
• [ ] QA whole flow as a n00b guest
• [x] "show" page for the mass invite that lists users? Or subcategory for mass-invited signups under "All Users" in admin
• [x] @ofsound can look at admin form (might be unifying forms across the site)

References #1088

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• ¹ It looks like all the flash related code in the users controller doesn't do anything because it's not show on the login page.

[Manfred]

Things that need some love:

• The public mass invite URL can be shorter
• The user form on the mass invite URL app/views/mass_invites_users/new.html.erb
• The mass invite archived warning page: app/views/mass_invites_users/archived.html.erb

[sudara]

Looking good! I'm going to edit the description with a few frontend-y todos.

[sudara]

🥳