Add tor loading progress UI

[diracdeltas]

Fix #14043

Test Plan:

Success case:

1. start brave
2. immediately open a tor private tab
3. you should briefly see a loading message in the URLbar
4. once loading is finished, you should be able to type in the URLbar

Fail case:

1. start an HTTP listener on port 9250 (so that Tor cannot bind to it.
   this causes Tor to not be able to connect).
2. start brave
3. open tor tab and wait for 20s
4. you should see an error message pop up
5. the button and link in the error message should work as expected

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).
• 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. (Ask a Brave employee to help if you cannot access this document.)

Reviewer Checklist:

• Request a security/privacy review as needed if one was not already requested.

Tests

• Adequate test coverage exists to prevent regressions
• Tests should be independent and work correctly when run individually or as a suite ref
• New files have MPL2 license header

[petemill]

Works nicely. We can refine styles once tor/0.23.x is rebased to include the style refresh 👍