Refactor resolvability checks based on type

app/models/concerns/issuable.rb

app/models/concerns/noteable.rb

app/models/concerns/resolvable_discussion.rb

app/models/concerns/resolvable_note.rb

app/models/diff_discussion.rb

app/models/diff_note.rb

app/models/discussion.rb

app/models/discussion_note.rb

app/models/individual_note_discussion.rb

app/models/legacy_diff_discussion.rb

app/models/legacy_diff_note.rb

app/models/note.rb

app/models/out_of_context_discussion.rb

app/services/issues/build_service.rb

app/views/projects/notes/_comment_button.html.haml

app/views/projects/notes/_form.html.haml