closes TryGhost/Ghost#6406
follow-up PR of #2
- adds a `timeZone` Service to provide the offset (=timezone reg. moment-timezone) of the users blog settings
- `gh-datetime-input` will read the offset of the timezone now and adjust the `publishedAt` date with it. This is the date which will be shown in the PSM 'Publish Date' field. When the user writes a new date/time, the offset is considered and will be deducted again before saving it to the model. This way, we always work with a UTC publish date except for this input field.
- gets `availableTimezones` from `configuration/timezones` API endpoint
- adds a `moment-utc` transform on all date attr (`createdAt`, `updatedAt`, `publishedAt`, `unsubscribedAt` and `lastLogin`) to only work with UTC times on serverside
- when switching the timezone in the select box, the user will be shown the local time of the selected timezone
- `createdAt`-property in `gh-user-invited` returns now `moment(createdAt).fromNow()` as `createdAt` is a moment date already
- added clock service to show actual time ticking below select box
- default timezone is '(GMT) Greenwich Mean Time : Dublin, Edinburgh, London'
- if no timezone is saved in the settings yet, the default value will be used
- shows the local time in 'Publish Date' in PSM by default, until user overwrites it
- adds dependency `moment-timezone 0.5.4` to `bower.json`
---------
**Tests:**
- sets except for clock service in test env
- adds fixtures to mirage
- adds `service.ajax` and `service:ghostPaths` to navigation-test.js
- adds unit test for `gh-format-timeago` helper
- updates acceptance test `general-setting`
- adds acceptance test for `editor`
- adds integration tests for `services/config` and `services/time-zone`
---------
**Todos:**
- [ ] Integration tests: ~~`services/config`~~, ~~`services/time-zone`~~, `components/gh-datetime-input`
- [x] Acceptance test: `editor`
- [ ] Unit tests: `utils/date-formatting`
- [ ] write issue for renaming date properties (e. g. `createdAt` to `createdAtUTC`) and translate those for server side with serializers
