[cr84] Settings page follow-ups: fixes guest mode, and refactors to overrides by module name

[petemill]

• Fix settings page in guest mode windows, use and override page visibility correctly
• Brings back the row divider lines for brave items.
• Refactor all settings webui overrides to be in modules with the name of the overriden component, as per the new README.md:

Use this directory to add modifications to any chromium polymer module.
Steps to take:
- Create a JS module which calls any of the polymer_overriding exported functions, e.g. `RegisterStyleOverride`.
- Import the module to the index.js inside this directory.
- Include the module in settings_resources.grd.

Fix brave/brave-browser#5065
Fix brave/brave-browser#2995

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)
• 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:

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.

[petemill]

might want to review commit by commit because the refactor commit confuses things due to moved files

[bsclifton]

Wow - I think I'll need to review the changes in Chromium 84 first... because that code changed quite a bit. This cleans it up even more; love it 😄 Will circle back after checking out C84

[petemill]

@bsclifton yes I did this refactor a bit iteratively:

• With the essential cr84 PR, I moved all the style definitions to component-specific JS files in style_overrides directory (through neccessity of having to remove the previous .html files), and made the API more declarative (RegisterStyleOverride vs the previous component naming convention brave-style-override-component-name).

• With this PR, I grouped all the overrides for a specific component (template, style, or behavior) so they are defined in the same place. Much easier to see what we're changing per-component as a whole. On reflection of what I did last week to get cr84 running, I decided this was the way I wanted to leave the cr84 conversion now that it's all JS.

[mkarolin]

++