Related Posts: set `font-size` for links

Set `font-size` for related post links to ensure consistent display
across themes.

Closes #485
Social Links: better handling of Customizer hooks

Social Links: better handling of Customizer hooks in non-admin context.

Create a `publicize_init()` helper function to load the Publicize
global object from a non-admin environment.

Merges
https://github.com/lancewillett/jetpack/commit/c059e06cf5d154c95a1c9d686
d9c76d4ed0612cf

Props @lancewillett @kovshenin

Fixes #906
Merge pull request #908 from Automattic/ticket/485,906

Ticket/485,906