User adds a Social Account to a Sock

[mattreduce]

Socks should be able to have many Social Accounts. Those could have attributes like these:

• url - string
• kind - string (will be used to display logo)

When adding an account, the form should let you select kind (maybe its label should be "Kind of account" or something) from a drop-down.

To start with, this drop-down could be populated with options that are hard-coded into the SocialAccount model, but later we could store them in the database and allow admins to manage that list. Will need to add SocialAccountKind records in db/seeds.rb at that point.