Add 2017 donation banner text

Here's the donation banner text for 2017, given to me by the crowdfunding team:

We had a good check-in with the end of year campaign team and finalized some taglines for you to work with.

    [Line 1, variable] <tagline>
    [Line 2, non-variable] Tor: Powering Digital Resistance.
    [Line 3, non-variable] Give today and Mozilla will match your gift!
    [Button] Donate Now!


The lines and the words in the button are hardcoded. This allows these strings to be translated, so we want to keep that, rather than make those lines part of the image. The downside of that is browsers will overwrite some font choices for those lines, so we should expect that system fonts will take over.

    The taglines we decided on are:
    Protecting Journalists, Whistleblowers, & Activists Since 2006
    Networking Freedom Worldwide
    Freedom Online
    Fostering Free Expression Worldwide
    Protecting the Privacy of Millions Every Day


So for example, the first would read:
Protecting Journalists, Whistleblowers, & Activists Since 2006
Tor: Powering Digital Resistance.
Give today and Mozilla will match your gift!
Donate Now! (Button)

We should get this text into the torbutton repository ASAP so that translations can be ready for the next release.