Implemented strong_parameters for Category/CategoriesController.

Category now requires parameters to be permitted by strong_parameters using #require or #permit for mass-assignment. Missing required parameters now throw a ActionController::ParameterMissing execption instead of the Discourse::InvalidParameters execption.
Implemented strong_parameters for Invite/InvitesController.

The email parameter is now required using strong parameters and will throw ActionController::ParameterMissing if it is missing. If the email address is incorrect or invalid, Discourse::InvalidParameters will still be thrown.
Implemented strong_parameters for PostAction/PostActionsController.

PostActionsController now uses strong_parameters' #require to require certain parameters. ActionController::ParameterMissing is now thrown when a reqired parameter is missing, rather than Discourse::InvalidParameters.
installdocs: Update with screenshots & more

* add screenshots from Ubuntu config process
* add instructions for updating predeployed VMs
* add details on configuring mail server
* don't install test gems into prod
* no need for bundle exec
* small cleanups
Implemented strong_parameters for Upload/UploadsController.

The topic_id param is now required using strong_parameters' #require method. If the parameter is missing ActionController::ParameterMissing will be raised instead of Discourse::InvalidParameters.
Admin flags UI shows when it's loading and when there are no results
Merge pull request #961 from Supermathie/sysadmin-docs

installdocs: Update with screenshots & more
Refresh admin user list after approval
Upgrade Ember to RC5. Disabled a deprecation warning that I believe is in error.
FIX: Couldn't navigate to Categories list via link
FIX: Compile templates properly with the latest handlebars
Add 'invite only' site setting
Hide registration on login modal if 'invite only'
Silently fail if user tries to sneak in

When 'invite only' is enabled, there's no way for a user to create an
account unless they try and sneak in by POSTing to /users/. We will
silently fail if this happens.
Disable OmniAuth account creation if 'invite only'
Don't require authentication for invites
FIX: Have onebox ignore internal links
Remove superfluous }
Merge pull request #965 from stephankaag/master

Correct invalid template
Merge pull request #962 from sanderdatema/dutch_translation

Updated Dutch translation