CodeMirror 6: Fix header token position and image fade-in

[personalizedrefrigerator]

Summary

This is a follow-up pull request to #58. It fixes issues related to the stylish theme and focus mode.

More specifically, it does the following:

• Corrects the position of # characters in headings in the stylish theme in CodeMirror 6.
• Corrects the opacity of img tags just below the active line.
• Removes reliance on .CodeMirror-line selectors (allowing them to be removed from the upstream pull request)
  • The upstream pull request had previously added the CodeMirror-line class to each visible line.

Screenshots

CodeMirror 5:

CodeMirror 6:

[CalebJohn]

Thanks!