Create an admin web page that shows messages from a given user

[fozziethebeat]

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Admins will need to review the messages posted by users. To make this simple, we should use the Backend API /api/v1/frontend_users/{username}/messages using the user's appropriate ID based on their auth method (for email: ID in the User table, for discord: userId in Account table).

This requires:

• [ ] A link from the user's entry in the main admin panel that goes to a new page (/admin/review-user/{id})
• [ ] Fields to set the number of messages to inspect and the start and end date.
• [ ] A table to display the fetched messages.
• [ ] A way to change the user's role assuming an admin might want to change their access rights after review.
• [ ] An API route to do the fetching from the Backend using /api/v1/frontend_users/{username}/messages. This layer should probably be the one that decides what user ID to send to the backend.

Blocked by #237