R1:6f90fbdef806

Send emails for email invites

Summary:
Ref T7152. Ref T3554.

- When an administrator clicks "send invites", queue tasks to send the invites.
- Then, actually send the invites.
- Make the links in the invites work properly.
- Also provide `bin/worker execute` to make debugging one-off workers like this easier.
- Clean up some UI, too.

Test Plan:
We now get as far as the exception which is a placeholder for a registration workflow.

{F291213}

{F291214}

{F291215}

Reviewers: btrahan

Reviewed…