Improve display of urls in profile

[drnlm]

#685 changed the profile to allow more flexibility with social profiles, but we should add some polish to the display.

At a minimum, we should display the profile entries as links (perhaps with some more validation on the entry to ensure they're well formed).

Although it would be nice to have the ability to specify custom validation and rendering functions, so we can duplicate the previous behaviour of asking for a twitter handle or a github uername and generating the correct url ourselves.