DEV: Add rubocop-rspec (#9288)

This adds rubocop-rspec, and enables some cops that were either already passing or are passing now, after fixing them in this commit.

Some new cops are disabled for now, with annotation: "TODO" or "To be decided". Those either need to be discussed first, or require manual changes, or the number of found and fixed offenses is too large to bundle them up in a single PR.

Includes:

* DEV: Update rubocop's `TargetRubyVersion` to 2.6
* DEV: Enable RSpec/VoidExpect
* DEV: Enable RSpec/SharedContext
* DEV: Enable RSpec/EmptyExampleGroup (Removed an obsolete empty spec file)
* DEV: Enable RSpec/ItBehavesLike
* DEV: Remove RSpec/ScatteredLet (It's too strict, as it doesn't recognize fab! as a let-like)
* DEV: Remove RSpec/MultipleExpectations

spec/models/site_setting_spec.rb

@@ -178,7 +178,7 @@ describe SiteSetting do
         end.to change { @fake_logger.warnings.count }.by(2)
 
         expect do
-          expect(SiteSetting.use_https(warn: false))
+          SiteSetting.use_https(warn: false)
         end.to_not change { @fake_logger.warnings }
 
         SiteSetting.use_https = false