Fix check for message media: avoid showing unneeded 'preview message'. Closes #798

* Message media check was always true. Fix to check media count
* No preview message only shows when there is something to preview.
Fix custom formsfield validation #671

* Actually return sane error messages
* Push errors directly into $post object
* Bind all error to 'custom_field' rather than trying to bind to
  individual fields as this didn't work and meant no errors were displayed
* Still a bit broken since only the last error will get displayed,
  not great if you have 10 errors and only see 1
* Fix unit test to grab the errors from the $post object
Merge pull request #1058 from aoduor/develop

Fix custom form fields validation #671
Clarify what facebook settings are for. Closes #1059
Merge branch 'develop' of git://github.com/ushahidi/Ushahidi_Web into develop
Fix incidents API returning spam comments. Closes #1002
Fix unicorn theme with man nav items. Closes #952
Fix redirect to addons/plugins when clean urls are off. Closes #1061
More changes - Ability to test keys and tokens on the twitter api
Merge branch 'develop' of https://github.com/ushahidi/Ushahidi_Web into develop