Introduce TextEditorView, which supports both Markdown and plain text.

MarkdownEditorView has been renamed and modified to support editing both
Markdown (through CodeMirror) and plain text (through <textarea>). This
edit mode can be set dynamically, allowing a user to choose how whether
a particular field is in rich text mode.

Currently, the default is plain text, which means that when used, all
editors for new objects will show a standard <textarea>. This will
change as I port things over to use the new API.

When placed in an inlineEditor, there will be an "Enable Markdown"
checkbox that the user may check to go in and out of rich text mode.

Reviewed at https://reviews.reviewboard.org/r/6465/