Backport backend work for time tracking.

app/assets/javascripts/lib/vue_resource.js.es6

app/assets/stylesheets/framework/variables.scss

app/helpers/issuables_helper.rb

app/models/concerns/issuable.rb

app/models/concerns/time_trackable.rb

app/models/timelog.rb

app/serializers/issuable_entity.rb

app/services/issuable_base_service.rb

app/services/slash_commands/interpret_service.rb

app/services/system_note_service.rb

db/migrate/20161223034433_add_time_estimate_to_issuables.rb

db/migrate/20161223034646_create_timelogs.rb

db/schema.rb

doc/user/project/slash_commands.md

doc/workflow/README.md

doc/workflow/time-tracking/time-tracking-example.png

doc/workflow/time-tracking/time-tracking-sidebar.png

doc/workflow/time_tracking.md

lib/gitlab/import_export/import_export.yml

lib/gitlab/time_tracking_formatter.rb