Set TinyMCE editor width/height

Ghost Userrequested to merge
fix/386-tinymce-editor-size into master

Closes #386 (closed)

Previous behaviour

Steps to reproduce:

  1. Go to event edit page
  2. HTML editor is really small

New behaviour

Steps to validate that it works:

  1. Go to event edit page
  2. HTML editor is large enough to easily edit text

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