Add Spell Checking (Option) in Plain Text Mode (Markdown)

[jens-br]

Currently, the plain text edit mode does not support spell checking:

The rich text edit mode already supports it:

It would be helpful if spellchecking could be activated in the plain text edit mode as well - at least as an option. :)

I guess the component used for this is in seahub/frontend/src/pages/plain-markdown-editor/index.js and https://github.com/haiwen/seahub/blob/master/frontend/src/pages/plain-markdown-editor/code-mirror.js
If I understand it correctly, the editor part of the view uses an underlying CodeMirror.
As a simple first solution, it should be possible to activate the browser-integrated spell checking for the CodeMirror by using the option spellcheck: "true" (c.f. codemirror/dev#63).