Accept invitations

[zhen0]

PR Checklist:

• add a short description of what's changed to the top of the CHANGELOG.md
• add/update tests (or don't, for reasons explained below)

Describe this PR

• Updates the design of the accept invitation page to match other onboarding pages
• Closes Update the design of the AcceptInvitation Page #282
• This page will only be shown to existing users who click on the emailed invitation link. New users will be shown invitations (where relevant) in the updated name-team page.
• We'll do a separate PR to update routing for onboarding (the accept route should be handled already by redirect route in auth nav guard)
• Also working on a separate PR to the docs to add information about teams and team switching in the UI

The page now looks like this:

[znicholasbrown]

This is looking really nice @zhen0! A few comments and also noticed this, which I don't think is intentional?

Changes made

[zhen0]

Thanks for the comments @znicholasbrown - changes made and ready for another review.

[zhen0]

Short addition to the team settings docs that we link to from this page in Prefect PR 3561

[znicholasbrown]

2 small changes, otherwise tested and LGTM @zhen0

Changes made!

[znicholasbrown]

LGTM @zhen0 😄