Allow full bug links in changelogs

[encukou]

Currently, Bodhi recognizes the shorthand (fedora|epel|rh|rhbz)#BUG_ID in changelogs and as a bug ID. I propose to accept a full URL like https://bugzilla.redhat.com/(show_bug.cgi\?id=)?BUG_ID as well.

• General-purpose editors/viewers are likely to make URLs clickable; and even in plaintext an URL can be pasted directly to a browser.
• It's more friendly to beginners: until you learn know what rhbz stands for, it's hard to find out.

I appreciate that this can be set in the config file, but a good default would be useful as well.

Would you consider a PR for this?