[New Rich Text Editor]: Completely replace the old editor

[habibayman]

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Desired behavior

Fully replace the old editor with the new editor 🎉

• The new editor should fit in
  four components:

• AssessmentItemEditor.vue

• AnswersEditor.vue

• AssessmentItemPreview.vue

• HintsEditor

• The new editor should be configured to replace the MarkdownViewer too

• Should have an equivalent behavior for the "minimize" behavior

Clean up:

• Remove all the code related to the old editor

Dependencies to be removed:

• @toast-ui/editor
• codemirror
• jquery
• showdown

Current behavior

At this point, the editor will be fully functioning properly in an isolated environment.
The old editor is still in its same place with all its related dependencies too.