Fix default-slide-preview-url

Fix #1246
Fixed #1235 (DeprecationWarning in utils.views.PDFView)
Fix link to list of speaker item on related items

fix #1242
Fixed #1158 (Clean up assignment.css)
PDF: Add missing 'fontName'.

And set 'registerFontFamily(...)' to use bold/italic in regular paragraphs.
Fixed #1104 (Enable copy/paste in dashboard widgets)
Replace own CKeditor with dedicated app
Merge pull request #1290 from ostcar/fix_overlay_on_preview

Fixed default-slide-preview-url.
Merge pull request #1292 from ostcar/fix_list_of_speaker_on_related_item

Fixed link to list of speaker on related items.
Merge pull request #1291 from emanuelschuetze/fix-1235

Fixed #1235 (DeprecationWarning in utils.views.PDFView).
Merge pull request #1295 from emanuelschuetze/pdf-fix

PDF: Added missing 'fontName'.
Merge pull request #1294 from emanuelschuetze/fix-1158

Fixed #1158 (Clean up assignment.css)
Merge pull request #1297 from emanuelschuetze/fix-1104

Fixed #1104 (Enable copy/paste in dashboard widgets)
Merge pull request #1278 from rolandgeider/ckeditor

Replace own CKeditor with dedicated app