removed Multipart class which is no longer needed

now handled with the restclient libraries
refactored out the preview from controllers

made more general as a shared method, and removed the duplication from many controllers
a general controller destory method for AssetsCommon

removed duplication and moved to AssetsCommon for those controllers that include it
unless the current destroy method is very specific
more generalisation and removing duplication of destroy method

currently the destroy is in it's own modules, but could eventually form a general
set of CRUD actions
shared some duplicated common behaviour in events_controller
some refactoring and tidying of avatar helper
fixes for tests
a little refactoring of content-type tidying up some complex methods
fixed tests that weren't web-mocked out properly

overzealous exception handling in content-blob was convering this up. Since being fixed a couple of tests were failind due to http calls not being mocked out with webmock (since the exception was previously being caught).
Now changed to mock out the http calls with a 404 error, which will produce the same error expected in the tests