"Back"-button can break after validation errors occurred when editing user profile

[Mathis-Z]

Steps to reproduce:

• Edit a user profile and enter some invalid data (e.g. invalid password)
• click "Save"
• click "Back" twice

After the validation error, your URL changes from /users/edit to /users. If you then click the "Back"-button twice it will go back to /users/edit and then to /users which is a route that does not exist.
The same happens at "users/sign_up".
A similar issue occurrs at "users/password/new".
This does not affect tasks, collections or groups.

[MrSerth]

Another issue with the back button occurs when editing a task (called "cancel" there). Since we are instructing the browser to do a full refresh (due to Turbolinks), the referrer is the edit page. Hence, going back simply reloads the page there.