Rich text formatting in question/step descriptions

[XanderVertegaal]

In the VWR prototype, some descriptions have numbered lists. I can imagine others using boldface for emphasis and other typesetting features too.

With TinyMCE implemented for StepInfo in #169, we can extend this feature to the description fields as well.

[EdoStorm96]

Tiny issue, mostly just add the TinyMCETextFieldMixin to the Question and Step forms in the admin. Currently the mixin works that it will override every TextField in a form, but we only want the descriptions to use TinyMCE. If it gets utilized in other non-description fields we'll have to change the mixin a bit. But try it out first.

Then you'll have to ensure the html which gets inserted through the mixin gets escaped in the frontend. Have a look at line 26 of FormSideBar.vue. I used Vue's v-html directive.