Move RSpec `shared_examples` from `spec/support`

Currently we have a bunch of files that define shared_examples in spec/support directory. Maybe it would be better to move those files to directories that those examples belong to?

We have:

support/mentionable_shared_examples.rb:shared_examples 'a mentionable'
support/mentionable_shared_examples.rb:shared_examples 'an editable mentionable'
support/project_hook_data_shared_example.rb:shared_examples 'project hook data' 
support/project_hook_data_shared_example.rb:shared_examples 'deprecated repository hook data'
support/taskable_shared_examples.rb:shared_examples 'a Taskable'
support/api/pagination_shared_examples.rb:shared_examples 'a paginated resources'
support/email_format_shared_examples.rb:shared_examples 'an object with email-formated attributes' 

For example, we can move shared examples for Mentionable to spec/models/concerns/mentionable/shared/mentionable.rb and spec/models/concerns/mentionable/shared/editable_mentionable.rb

Then we can also use something like below if those examples are needed:

require 'models/concerns/mentionable/shared/editable_mentionable'

Ideally it would be nice if support/ directory contained only configuration files for test harness. What do you think @DouweM @dzaporozhets @rymai?