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.
Javascript uses lazy evaluation, which means that function definitions aren't looked up until the function is called. As a result, there's no technical reason why the function definition needs to be before the function call in terms of line numbers, as long as the function call is executed afterward.

However, for reasons of clarity, the function should *absolutely* be defined first in the file, unless there is some other reason why it can't or shouldn't be. This code will probably run just fine, but if it's confusing, then it needs to be fixed so that it's not confusing anymore.