Remove uses of html imports from WebUI

[petemill]

Chromium 86 no longer supports html imports, even for WebUI. We were only using it to:

1. Import a polymer component, br-toolbar. This has already been converted to an es module, so we simply include that.
2. Import cr.html which was never neccessary because we also include cr.js already. cr.html only additionally adds the import for PromiseResolver, which no brave code uses.
   TODO: remove calls to cr.js and just import * as cr from cr.m.js directly wherever cr.* is used.

Fix brave/brave-browser#11937

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
  • Android
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that these changes pass automated tests (unit, browser, security tests) on
  • iOS
  • Linux
  • macOS
  • Windows
• Verified that all lint errors/warnings are resolved (npm run lint, npm run gn_check)
• 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
• Public documentation has been updated as necessary. For instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-compatibility-issues-with-tracking-protection
  • https://github.com/brave/brave-browser/wiki/QA-Guide

Test Plan:

Ensure brave/brave-browser#11937 is resolved. Additionally ensure the following WebUI pages still work correctly:

• Rewards Tip
• Rewards Internal
• Ad Block
• IPFS
• Webcompat Reporter

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

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on.
• All relevant documentation has been updated.

[emerick]

LGTM

[bsclifton]

LGTM too - compiling so I can give it a go (can report back; don't hold up the merge for me)

[bsclifton]

Confirmed- works great! 😄 🎉