Add about field to preprint_providrs model [OSF-7409]

[sheriefvt]

Purpose

On branded preprints, communities need a way to give 'About' information to users. There is a need to add a link in the footer, text is 'About' and is a link to their about information. A new field in the model need to be added to store the about page URL and the Populate Preprint Providers script needs to be updated to include the contents of this field for all existing providers.

Changes

• Add new field to the model
• Update serializer
• Run migrations

Side effects

Ticket

https://openscience.atlassian.net/browse/OSF-7409