UserCSS mode, fixed USO double-install, CM 5.31

• CodeMirror 5.31.0 - it fixes inability to copy more than 1000 characters
• Fixed userstyles.org double-firing of install/update events
• Fixed find-styles link for extension themes on userstyles.org
• Fixed exporting of a very large database in the style manager
• UserCSS mode
  • Stylus can install from URLs that end with .user.css or .user.styl
  • Style variables can be tweaked in style manager
  • Write new style button may be configured to open in usercss mode: enable it in the Options -> Advanced block
  • documentation: https://github.com/openstyles/stylus/wiki/Usercss
  • sample: https://gist.githubusercontent.com/eight04/1b9edeb170d9f8bbabfb06dc6627f8f7/raw/foo.user.css
• Firefox's built-in XML document viewer is styled correctly now
• Firefox before 57: reliable detection of browser version as it could be spoofed by a legacy addon
• Import from Mozilla-format in the editor can handle more edge cases
• Popup's "Turn all styles off" label is highlighted in red when enabled
• Various edge-cases fixed/tweaked