Fix #8096: Add blocked page interstitial

[cuba]

Summary of Changes

This pull request fixes #8096

When blocking document type requests we can intercept them and show a nice error message instead of a blank screen. Later we can add a "continue anyways" button but that is a little more complicated because it requires rewriting content blocker rules and likely some brave core changes if we want to store the exceptions on that side. So I left this out for now.

Could also generalize the CSS (and perhaps even the html) as it follows the latest designs which will need to be used for our other interstitials:

security review: https://github.com/brave/reviews/issues/1474

Submitter Checklist:

• [x] Unit Tests are updated to cover new or changed functionality
• [x] User-facing strings use NSLocalizableString()
• [x] New or updated UI has been tested across:
  • [x] Light & dark mode
  • [x] Different size classes (iPhone, landscape, iPad)
  • [x] Different dynamic type sizes

Test Plan:

1. Go to adcash.com
2. Ensure the interstitial page is appearing

Screenshots:

Reviewer Checklist:

• [ ] Issues include necessary QA labels:
  • QA/(Yes|No)
  • bug / enhancement
• [ ] Necessary security reviews have taken place.
• [ ] Adequate unit test coverage exists to prevent regressions.
• [ ] Adequate test plan exists for QA to validate (if applicable).
• [ ] Issue and pull request is assigned to a milestone (should happen at merge time).

[iccub]

Tests seems to need a fix now