feat: Provide a way to change institutions from the UI

• adds /change_institution
• adds /user/<username>/institution endpoint
• fixes up static rendering (using url_for instead)
• adds a 403 page in case we don't have the right permissions to change for the provided user (e.g. user tries to change it for someone else)
• changes the pop-out for the user information to display institution instead of username, and make it linkable for being able to change it