Plugin installation prompt: add link to approved plugin spreadsheet

[tmmbecker]

Describe the bug

The prompt as it is now is helpful but people don't know that the spreadsheet exists so they're not looking for it. This has increased the number of review requests but in many cases, the plugins have already been reviewed. So we're having to follow up to tell people what they should already be able to find out for themselves.

If we can't get to this in the next release, I'd like for us to revert this change until we can figure out a better process. As it is, it's making more work than it's saving.

Labeled it a bug because the messaging isn't as sufficient as it should be to make this prompt effective.