Make brave unusable to encourage updating

[diracdeltas]

This PR replaces the newtab page with the notice to update Brave and makes the urlbar unusable.

Test plan:

1. Launch Brave without brave-core installed
2. You should see a notice to update brave.
3. Clicking on it should download Brave.
4. The urlbar should not be usable.
5. With brave-core now installed, launch the old Brave again.
6. Now you should see a notice to launch the newer Brave installation.
7. The urlbar should still be unusable.

Test plan for disabling tab restoration:

1. Open previous version of muon brave and open some tabs/windows
2. Update to this version
3. You should not see any of your tabs/windows restored

Screenshot:

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.)

Test Plan:

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

[diracdeltas]

just realized, i should disable tab restoration too

[diracdeltas]

tab restoration disabling is done now

[bsclifton]

@diracdeltas you can still open private/Tor tabs, which don't show our new messaging

Should we disable these too?

[diracdeltas]

you can still open private/Tor tabs, which don't show our new messaging

good catch, will disable private tabs

[diracdeltas]

@bsclifton changed it so that private/tor tabs now also show the notice