Add approval workflow for paid registrar requests

[cmsetzer]

This builds upon our work in #3595 and #3631 to add an approval workflow for sign-up requests by paid registrars (a.k.a. "firms"). An approval workflow already exists for library sign-up requests; this branch generalizes the library approval form so it supports firms as well.

Before

• User submitted a paid registrar request at https://perma.cc/sign-up/firms
• Perma admins then emailed them, going back and forth a few times to get registrar info and set a base rate
• Perma admins then used the Perma DB admin interface to manually create a registrar, populate fields, and assign the base rate

After

• User submits paid registrar request at https://perma.cc/sign-up/firms
• A pending registrar is automatically created in the Perma DB with registrar name, email, etc. populated
• An email is sent out to user and Perma admins with all the field data supplied by the user
• Using the approval link in the email (or by clicking a button on the Manage Registrars screen), Perma admins can approve/reject the registrar request and change the base rate in one place
  • As we've discussed, if the pending registrar does not already have a registrar user associated, Perma admins must assign a registrar user before they can approve/reject the registrar. A user search interface is provided on the approval page

[codecov]

Codecov Report

Attention: Patch coverage is 83.76068% with 19 lines in your changes missing coverage. Please review.

Project coverage is 69.11%. Comparing base (3aba690) to head (8567f78).
Report is 13 commits behind head on develop.

Files with missing lines	Patch %	Lines
perma_web/perma/views/user_sign_up.py	79.31%	12 Missing ⚠️
perma_web/perma/forms.py	80.00%	7 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3659      +/-   ##
===========================================
- Coverage    69.64%   69.11%   -0.53%     
===========================================
  Files           54       54              
  Lines         7339     7467     +128     
===========================================
+ Hits          5111     5161      +50     
- Misses        2228     2306      +78     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

---

🚨 Try these New Features:

• Flaky Tests Detection - Detect and resolve failed and flaky tests
• JS Bundle Analysis - Avoid shipping oversized bundles

[rebeccacremona]

This looks great! This is going to be such a helpful feature!!

A couple thoughts related to searching:

• great idea to re-use the user list markup

• I think if no search results are found, it's a little confusing, it looks like the form maybe didn't submit

• I think the experience of submitting the search form could be frustrating for keyboard users and screen reader users, since focus will reset to the top of the page upon form submission. Since this is an internal, admin-only page, and since this page isn't currently set up to have any javascript... it's probably not worth the effort to address now. But, if we ever get around to redesigning this part of the app, good for us to think about.

LGTM, left a few remarks and questions

[rebeccacremona]

I would like to officially apologize for these tests.

[cmsetzer]

Thanks for the review @rebeccacremona! I fixed the issues you raised as well as a couple other small bugs I discovered. Going ahead and merging this.