Feature/unified social links

# Purpose
* Solves #1227

# How
* Moves all social link/field definitions to `framework.auth.core.User`
* Creates the field `SOCIAL_FIELDS` which defines all social fields and how to format their URLS
* `personal_site` is now referred to as `personal` everywhere

# Possible side effects
* Needing to invoke `migrate_search`
Can you please add a basic unit test for this property?