Fix server-side validation

Closes #3122
-Fix validation so that all values are validated instead
 of just values that evaluate to true.
-Ensure validation methods consistently return promises
 and switch error handling from try/catch to promise.catch
 to get rid of unhandled rejection warnings.
-Add 0 and 1 to list of acceptable values in boolean validation.
Allow user to accept invitation

closes #3081
- added route `/ghost/api/v0.1/authentication/invitation`
- added accept invitation
- added signup with token
- removed check() from users api
- fixed promise in resetPassword()
Indirect post title in editor

Closes #3179
- Add `titleScratch` property to `PostModel`.
- Changed references to `title` to `titleScratch`
Added fadeout to gh-popover

Closes #2868
- Uses jQuery's `.fadeOut` whenever a popover is closed.

- Reordered `gh-popover`'s code into something a bit more logical and, if
  I may, pretty
- Renamed `open` property into `isOpen`. `isOpen` should only be
  manipulated via `close()` and `open()`
- Added `closing` property to help track state in the case of rapid clicks
  on a popover's button, allowing us to abort
` Added `open()` function
Merge pull request #3124 from jaswilli/model-validation

Fix server-side validation
Update Ghost-UI version to 0.8.2

References #3103 #3155 #3084
Merge pull request #3178 from novaugust/popover-fadeout

Added fadeout to gh-popover
Merge pull request #3192 from sebgie/issue#3081

Allow user to accept invitation
Merge pull request #3194 from novaugust/title-scratch

Indirect post title in editor
Merge pull request #3198 from PaulAdamDavis/master

Update Ghost-UI version to 0.8.2