Legislator Profile - Account Verification Flow

## Summary

As part of the Legislator Profile project, we want to allow users to create new `LEGISLATOR` type accounts associated with a specific Legislator that's permitted to add information on their behalf. To that end, we want to add a verification step to ensure the profile is actually being created by someone connected with the legislator in question (to avoid false representation).

The account signup process is covered in a separate issue - this issue covers the admin flow to approve/reject LEGISLATOR profile requests. This should function similarly to the (currently disabled) flow for ORG profiles.

## Success Criteria
- [ ] Add `PENDING_LEGISLATOR` accounts to the view on the Upgrade Request Admin page
- [ ] Allow admins to approve or reject these requests
- [ ] An `approved` request upgrades the account from `PENDING_LEGISLATOR` to `LEGISLATOR`
- [ ] A `rejected` request downgrades the account from `PENDING_LEGISLATOR` to `USER` (See open questions)
- [ ] After the admin decision, send an email to the user in question informing them that they were either approved (with a link to the legislator profile page) or rejected (with the admin-provided reason for the rejection).

## Open Questions (for @mvictor55 )
- If we reject a legislator account request, should we even allow the account to remain up (because it's presumably someone trying to impersonate a legislator), or should we just delete the account entirely?

## Screenshots

Current Admin page for Pending Upgrades:

## Additional Links
- Mockup for Legislator Profiles: https://maple-leg-profiles.netlify.app/