Lots of refactoring and efficiency improvements

Lots of refactoring and efficiency improvements

• Refactor browser_launcher.py.
  -- (Get ready for deprecations coming in selenium 4.)
• Have assert_text_not_visible() return True if successful.
  -- (It still fails the test if the unexpected text is still visible.)
• Refactor seleniumbase resource files:
  -- Save space by moving copies of resource files out of the repo.
  -- (Most JS files were already getting retrieved via CDN link.)
• Refactor seleniumbase imports.
  -- (Defer importing packages until needed, which saves time.)
• Update non-required dependencies:
  -- pyflakes==2.3.1;python_version>="3.5"