Add "Manage custom filters" context menu item

[antonok-edm]

Resolves brave/brave-browser#14978

I've also added 2 commits here to clean up minor details from #8156 - one function no longer needs to be async and there are a bunch of locale strings that can now be removed safely.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

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, 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-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

From any page, right click on the page and check that the Brave > Manage custom filters context menu item exists and navigates to a new brave://adblock tab, or an existing one if present.

[bsclifton]

Functionality works great! Just had a question about the async keyword removal on addSiteCosmeticFilter. The other changes look good! 😄

[antonok-edm]

@bsclifton after discussion with @pes10k it looks like the async change shouldn't affect anything - onSelectorReturned is only used as the callback in chrome.tabs.sendMessage(tabs[0].id, { type: 'getTargetSelector' }, onSelectorReturned), and the return value of that callback is ignored as per https://developer.chrome.com/docs/extensions/reference/tabs/#method-sendMessage

[bsclifton]

@antonok-edm perfect - thanks for confirming. Sorry I didn't get to dig in there more after raising the comment. LGTM! 😄👍

[bbondy]

Did this possibly introduce this CI failure on unittests @antonok-edm ?
https://ci.brave.com/blue/organizations/jenkins/pr-brave-browser-backup-search-network-pre-init/detail/pr-brave-browser-backup-search-network-pre-init/9/pipeline/

[antonok-edm]

Yep, my fault - sorry about that! 😬

I just opened #8429 to update the tests accordingly.

[bsclifton]

ah crap - my bad on that 😱 I saw the pre-init failure and (unfortunately) assumed it was an npm vulnerability. Great catch @bbondy 😄