[PR] Allow entry of arbitrary classes for sections

* Provides a text input for space delimited classes to be arbitrarily added to `section` elements
* Cleans the data to avoid broken markup
* Outputs space delimited classes in the front end markup

for

* [x] Single template
* [ ] Sidebar Right
* [ ] Sidebar Left
* [ ] Halves
* [ ] H1

Will be ready tomorrow morning for deployment