Allow admins to customize "social media" options

[Aaron Wells]

If you go to "Content -> Profile -> Social media", you can provide a list of your usernames in various social networks. We've also provided an "other" option to let this encompass additional types of social networks. But! It's still a hard-coded list. The important networks will shift over time, and depending on the circumstances of the institution running Mahara.

When we updated this from the previous system (which listed mostly instant messaging systems: ICQ, MSN, etc), we stayed with a hard-coded system because it was easier to do. But in the long run we need to make this system flexible, to allow admins to customize the options listed here.

This will be challenging because the hard-coded system allows us to also generate custom URLs for each system, and provide custom icons for them. With a custom system, I suppose we'd want to provide some kind of variable-based URL generator, and/or maybe fetch the favicon for the domain name?