Buggy behaviour when trying to message missed contact request by yourself

[rashfael]

Steps to reproduce:

1. Be staff of an exhibitor
2. click "Contact Exhibitor" and let prompt expire
3. Go to /contact-requests and click MESSAGE on your own missed contact request by yourself

What happens:
a. if you have an existing DM channel joined, the first DM will open (because it searches for a DM channel with yourself in it and 2 users)
b. if you have no DM channels joined, an actual channel will be requested and the server will respond with chat.denied

What should happen:
Either we allow self-dms, show an error when trying to open a self-dm or prevent pressing the message button at all, when you try to contact yourself.

Sidenote: You cannot DM yourself via the normal means of opening a dm with + in the sidebar.