Externalize HTML/JS changes to extensions item_list.html/js.

[mkarolin]

• Minimizes patch of extensions' page item_list.html/js by externalizing our changes into a separate dom-module,
• Adds brave_extensions_resources.pak.

Fixes brave/brave-browser#3012

Submitter Checklist:

• 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).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests) on
  • Windows
  • macOS
  • Linux
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

Copied from #1358:

• Start Brave with a fresh profile
• Navigate to brave://extensions
• Verify that the page shows text Find extensions and themes in the [Web Store]
• Verify that clicking on the link takes you to the Chrome Web Store
• Add one or more extensions
• Return to brave://extensions page
• Verify that the page shows text Find extensions and themes in the [Web Store],
• Verify that clicking on the text takes you to the Chrome Web Store
• Return to brave://extensions page
• Click on Details button for any extension installed from Chrome Web Store
• On the details page, verify that the Source shows text Web Extensions Store

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[bsclifton]

Changes LGTM! 👍 All tests/linting passed, built without issues. Ran through manual test steps (pulled them into this PR)- everything worked great 😄