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/mailers/user_notifications_spec.rb

@@ -968,7 +968,7 @@ describe UserNotifications do
   end
 
   # notification emails derived from templates are translated into the user's locale
-  shared_examples "notification derived from template" do
+  shared_context "notification derived from template" do
     let(:user) { Fabricate(:user, locale: locale) }
     let(:mail_type) { mail_type }
     let(:notification) { Fabricate(:notification, user: user) }