Limit partner suggestions to eligible users

[Samwalton9]

Description

Added a check that the user is valid (500+ edits, 6+ months, not blocked) to post a suggestion, as a spam-limiting measure. Didn't check recent activity, because it seems fine for a user who is just inactive to suggest or upvote.

Rationale

We were getting a lot of spam on that page from newly-registered WP accounts, this should stop that.

Phabricator Ticket

[//: # (Link to the Phabricator ticket)](https://phabricator.wikimedia.org/T415249)

How Has This Been Tested?

Added new tests, and tested locally with both an eligible and ineligible account.

Screenshots of your changes (if appropriate):

Types of changes

What types of changes does your code introduce? Add an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Minor change (fix a typo, add a translation tag, add section to README, etc.)

Assisted with Claude Opus 5.5 (I'm trying it out for this repo, it worked relatively well!)