Fixes: staying signed in, sending requests, and profile changes

Three things members hit this week, all fixed.

Signed in but told to sign in. If a page had been open for a while, actions could fail with "You must be signed in to do that" while you plainly were. Your session token was expiring in the background and being sent anyway. It now refreshes before it is used.

Mentorship requests failing. Sending a request could fail with a message asking you to try again -- and trying again could never have worked, because the request was checking the mentor against something that did not exist. Fixed, and covered by a test so it cannot come back quietly.

Profile changes not appearing. Updating your profile could leave your own profile page showing the old version for up to a day, while the home page showed the new one. Saving your profile now clears that straight away.