Fix for STUD-2097

This pull request fixes https://openedx.atlassian.net/browse/STUD-2097.

The reason behind the issue was the fact that default text editor font set in tinymce-studio-content.css was not among the list of fonts tinyMCE use out of the box.

The solution is twofold:
1. Prepended font-family from tinymce-studio-content.css to font selection dropdown.
2. Added a button to clear font-family style only - useful to keep styling in resulting html minimal.

Unfortunately, such non-functional customizations are very hard to test. There were some analogous modifications already, and I was unable to locate any tests for them. 

So, if there exists some kind of mechanism for testing such modifications, I would be grateful for any guidance and will do my best to use it.