Make "Not now" the default for HTML5 canvas access permission dialogue.

This alters the `PopupNotification`s [0] which notify users that a
website has attempted to access an HTML5 canvas. The new default
ordering for buttons is:

    Not now (default)
    Never for this site
    Allow in the future

 * FIXES #12684. [1]

NOTE: Applying this patch requires an additional patch to TorButton, to
store the additional UI strings before localisation. [2]

[0]: https://mxr.mozilla.org/mozilla-esr24/source/toolkit/modules/PopupNotifications.jsm
[1]: https://bugs.torproject.org/12684
[2]: https://github.com/isislovecruft/torbutton/commit/368e74d62df349b27cf578525c3fa15da19ccdc2