[#797] Change password through API

The REST API has been updated with a new view on the usel model: /rest/v1/user/{pk}/change_password. A POST request to this view with the old_password, new_password1 and new_password2 will check if the credentials are valid and will update the user password.

Also, the form and view in MyRSR for updating a user password has been updated, now using the API through jQuery.
[#797] Merged with Dan's template work
[#797] Merge branch 'feature/835_v3_my_details' into feature/797_v3_change_pw
[#797] Code cleanup
[#797] Updated change password through API

- Implemented a PermissionDenied when a user tries to change another user's password
- Placed the CSRF javascript code in a separate file
- Some small style issues
- Removed the max_length for passwords
Merge pull request #801 from akvo/feature/797_v3_change_pw

[#797] Change password through API