Plugin to customize the theme's appearance and branding.

**Purpose**
Customize the logo, styles and colors of the theme.

**Use case**
- A team or an org wants to make D.T feel and look more like other products they currently use.
- For more security aware contexts: changing the logo to make the theme less easily identifiable. A passer by seeing the name or logo and then googling it.

**Requirements**
- A plugin with settings pages in the wp-admin that allows a D.T admin to customize the appearance.
- customize the logo in the nav bar
- customize the logo on the login page (not displayed by default)
- change primary color and secondary colors.

**Code considerations**
In many cases the colors are currently hard coded. These need to be refactored to a css class or set of classes.

**Deeper security**
This issue only pertains to the surface appearance of the theme. A look at the html and css will reveal that it is a wordpress site using disciple-tools-theme. This is discussed more here: https://github.com/DiscipleTools/disciple-tools-theme/issues/615