Hold first tab load until sheilds are ready

[cuba]

Summary of Changes

This pull request fixes #6261

Tab loading awaits the preparation of some ad block data. To achieve this I had to rework how the UI is loaded on the BVC so that the elements are loaded but only "enabled" once ABD is ready.

Submitter Checklist:

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

Test Plan:

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).

[cuba]

Merged here: #6417