Prevent duplicate separators for all context menu items.

[bsclifton]

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Ran git rebase -i to squash commits (if needed).

This update will ensure we will never have double separators going forward on Windows or Linux.
Fixes #5765

Auditor: @bbondy, @cezaraugusto

Includes these changes:

• Added new sanitizeTemplateItems method to filter out bad entries
• Moved menuUtil from app/browser => app/common
• Moved some tests to match the new directory structure (ex: test/unit/app, instead of test/unit/lib)
• Rename template functions in menuUtil and items in contextMenu.js to reinforce distinction between template items and electron menu items (the compiled template)
• The new sanitize method is called in each method which builds a context menu

Test plan

• Launch Brave on Windows or Linux
• Right click on each of the about pages and ensure there are no double separators (might be easy to go to about:about to launch each)
• Try other context menus
  • Right clicking on a tab
  • right clicking on page
  • right clicking a bookmark / bookmark folder in the bookmarks toolbar
  • right clicking inside the downloads bar (shown after downloading an item)

[bbondy]

++ great stuff, I thought about doing this before as well.

[bsclifton]

removed milestone