Add handling for huge profile picture uploads

[Atyanar]

Currently the generic "We're sorry, but something went wrong." message is thrown when a user tries to upload a picture that is larger than 1 MB.
The picture could be automatically scaled down or a more intuitive error message could be shown.

Excerpt of the stack trace:

13:57:41 web.1 | I, [2016-04-14T13:57:40.883853 #177] INFO -- : Completed 500 Internal Server Error in 279ms (ActiveRecord: 8.5ms)
13:57:41 web.1 | F, [2016-04-14T13:57:40.887394 #177] FATAL -- :
13:57:41 web.1 | ActionView::MissingTemplate (Missing template users/edit, application/edit with {:locale=>[:de, :en], :formats=>[:html], :variants=>[], :handlers=>[:erb, :builder, :raw, :ruby, :slim, :coffee, :jbuilder]}. Searched in:
13:57:41 web.1 | * "/mammooc/app/views"