Replace Ace with CodeMirror 6 for the Custom CSS editor

Ace 1.3.3 hid the CSS from screen readers and trapped the Tab key,
causing accessibility issues.

• Adds cssedit/, a CodeMirror 6 editor that make ui bundles with esbuild into static/js/cssedit.bundle.js
• Labels the editor and describe its keys: Tab indents; Esc then Tab, or Ctrl+M (Shift+Option+M on macOS), moves focus out
• Keeps Ace's Ctrl+D / Cmd+D shortcut for deleting a line
• Previously, mobile fell back to the plain textarea. Now the editor loads on all devices, and falls back to the plain textarea if the bundle is missing
• Removes Ace

Fixes #1776