[#760] Added new page for changing passwords in MyRSR

No link to the password changing page is included on the main page of MyRSR yet, but page can be visited at ../myrsr/password_change
[#760] Updated change password page

The change password page has been updated, now using an AJAX call and giving feedback. However, same as for the other AJAX calls, the UI and UX still need improving.
Merge pull request #762 from akvo/feature/760_password_page

[#760] Added new page for changing passwords in MyRSR