FR: Allow custom social types

[AndrewHaine]

In the Socials section we have a dropdown with a number of existing social media platforms.

We need to allow people to add their own items which will get merged into this list before outputting in the SuggestMode.

I envisage this working like this:

• We’ll add an additional field in the Addon Settings page allowing users to define additional social link types
• These link types will then be merged in with the default ones as part of the SuggestMode
• Items will be ordered alphabetically and pulled through to the socials table

[AndrewHaine]

Added in #42