Update tests and use js-issuable class for context form

app/assets/javascripts/issue.js.coffee

app/assets/javascripts/merge_request.js.coffee

app/views/projects/issues/_issue.html.haml

app/views/shared/issuable/_context.html.haml

spec/features/issues_spec.rb

spec/features/task_lists_spec.rb

spec/javascripts/fixtures/issues_show.html.haml

spec/javascripts/fixtures/merge_requests_show.html.haml