Improves a sentence in guides/security

Changed "... books make this wrong" to "... books get this wrong"
Merge pull request #12167 from hannesfostie/grammar-improvement

[ci skip] Improves a sentence in guides/security
Remove BasicRendering and remove template functionality from AbsC::Rendering
Remove remaining coupling with AV in MimeResponds
Remove BasicRendering tests
Added Loofah as a dependency in actionview.gemspec.
Implemented ActionView: FullSanitizer, LinkSanitizer and WhiteListSanitizer in sanitizers.rb.
Deprecated protocol_separator and bad_tags.
Added new tests in sanitizers_test.rb and reimplemented assert_dom_equal with Loofah.
Removed duplication in the deprecated methods.
Added PermitScrubber which allows you to permit elements for sanitization.
Reordered form removal with stripping.
Removed the contains_bad_protocols? method as well as the tests for it. Loofah already deals with this.
Changed expected value from '<b>' to empty string.
bad_tags include form since we remove it. Also to prevent a should_allow_form_tag test creation.
Added guard clauses to FullSanitizer.
Extracted failing tests in santiizers_test.rb into their own methods and marked them as pending.
Added video poster sanitization testing (from @vipulnsward).
Changed the implementation of assert_dom_equal.
Renamed the SanitizerTest class to SanitersTest, to remove the conflict with the old SanitizerTest for html-scanner.
Restructured the implementation of assert_dom_equal and assert_dom_not_equal.
Changed the dom.rb to actually compare elements! Extracted more things and added documentation for new methods.
Added removal of script tags to WhiteListSanitizer.