Fixes cache problem with private tabs for siteSettings

[NejcZdovc]

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.

Resolves #10829

Auditors:

Test Plan:

• visit one site with ads
• visit the same site in a private tab
• Allow Ads and Tracking in this private tab
• check that ads are loaded in the private tab
• visit the regular tab and reload it
• make sure that ads are still blocked in the regular tab

Reviewer Checklist:

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

[NejcZdovc]

travis is passing now for test Bravery Panel Adblock stats iframe tests detects blocked elements in iframe in private tab

[codecov-io]

Codecov Report

Merging #10836 into master will decrease coverage by <.01%.
The diff coverage is 41.66%.

@@            Coverage Diff             @@
##           master   #10836      +/-   ##
==========================================
- Coverage   54.16%   54.16%   -0.01%     
==========================================
  Files         247      247              
  Lines       21548    21554       +6     
  Branches     3338     3338              
==========================================
+ Hits        11672    11674       +2     
- Misses       9876     9880       +4

Flag	Coverage Δ
#unittest	54.16% <41.66%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
app/filtering.js	18.58% <0%> (ø)	⬆️
app/common/cache/braverySettingsCache.js	52.38% <50%> (-7.62%)	⬇️

[diracdeltas]

works great. thanks for catching this.