Allow user configurable extensions

We will mostly be using this for team meetings/collaboration, and so it is nice to allow users to enter their own pre-arranged (and memorable) extension numbers for specific teams/meetings, rather than have to enter the random 5 digit ones each time. The attached patch does just this. If the extension field is left blank then the number is auto-generated as before. Note that this doesn't make sense to include on the content type settings pages, as the extension is inherently unique to each meeting. I patched the default views also, but they didn't seem to be working for me to start with (also, there are duplicate files it seems).