Some fixes in error handling of editing users and posts

This pull request contains the following changes:
* Handle user edit errors with proper error messages. Each input field will have it's own error message as intended, but the error was not flashed.
* HTML for the error messages next to input fields was wrongly escaped in create_edit views for users and posts. This is now fixed and messages are displayed as expected.
* In Bootstrap 3 the classname for errors on .form-group are named "has-error", not "error". This is now fixed and the labels will now be displayed with the intended style (e.g. red).
* Indent style in the two create_edit views was mixed (tabs/spaces), now it's only one indent type.
* Changed use of class from "Url" to "URL" in Post model, which cleared the question in comment at top of Post model file (quote: "not sure why i need this here :c")