Backport timetracking frontend to CE.

app/assets/javascripts/diff_notes/diff_notes_bundle.js.es6

app/assets/javascripts/issuable/issuable_bundle.js.es6

.../issuable/time_tracking/components/collapsed_state.js.es6

.../issuable/time_tracking/components/comparison_pane.js.es6

...suable/time_tracking/components/estimate_only_pane.js.es6

...ripts/issuable/time_tracking/components/help_state.js.es6

...issuable/time_tracking/components/no_tracking_pane.js.es6

.../issuable/time_tracking/components/spent_only_pane.js.es6

...pts/issuable/time_tracking/components/time_tracker.js.es6

...cripts/issuable/time_tracking/time_tracking_bundle.js.es6

app/assets/stylesheets/framework/variables.scss

app/assets/stylesheets/pages/issuable.scss

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

app/views/projects/merge_requests/_show.html.haml

app/views/projects/merge_requests/conflicts.html.haml

app/views/shared/icons/_icon_stopwatch.svg

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

config/application.rb

spec/features/issues/user_uses_slash_commands_spec.rb

spec/javascripts/issuable_time_tracker_spec.js.es6