Footer Email Reference

## Feature 
Increase maintainability of references in footer

## Affected Area
[footer](https://github.com/Code-dot-mil/code.mil/blob/master/src/_data/footer.yml), lines 58 & 59

## Description
'href' for email uses a liquid reference to the configuration file. However, the associated 'text' is hard-coded.  hard-coding could be replaced with: 

```
'{{site.email}}'
```