Make `gfm_auto_complete` into a module and fix up tech debt

app/assets/javascripts/dispatcher.js

app/assets/javascripts/gfm_auto_complete.js

app/assets/javascripts/gl_form.js

app/assets/javascripts/issuable_form.js

app/assets/javascripts/main.js

app/assets/javascripts/notes.js

app/views/layouts/_init_auto_complete.html.haml

app/views/layouts/application.html.haml

spec/features/projects/gfm_autocomplete_load_spec.rb

spec/javascripts/gfm_auto_complete_spec.js