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.
Here I just followed the already established pattern. There are helper functions like use_plugin that defined after their first usage. I believe the intention was to have all helper functions defined at the bottom, but later more steps were added. I would actually move all the helper functions to the bottom, but decided not to do so to minimize diff. But since we're discussing this, do you mind having all the helper functions at the bottom? I believe it's a common habit of placing actual test functions at the top.