Move non theme related code to the plugin

[tobeycodes]

The README mentions:

Functionality should be built into the 10up must-use functionality as much as possible. Presentation should be kept in the theme. Separating these two makes long term development, maintenance, and extensibility much easier.

However there are several places in the theme which have code not related to presentation. Moving this code makes it easier for projects with multiple themes to share code and share a mu-plugin rather than duplicate code and increase maintenance effort.

https://github.com/10up/wp-scaffold/blob/trunk/themes/10up-theme/includes/overrides.php