donate: change bitcoin url to https to avoid a redirect; host flattr image ourselves

RE: flattr: Since we force https on the internet facing site, with the http
link there was a warning from the browser about 'mixed mode content' . Using
https://api.flattr.com/button/button-static-50x60.png caused the image to
disappear if one only accepts strong ciphers in the browser
(e.g.  security.tls.version.min set to 2 in Firefox). It would look like
we were doing something wrong even though it's not us, so that's not a viable option.

Looking over https://flattr.com/terms it seems that there's no problem with our
hosting it ourselves. It's also a tiny image besides.

Now to email flattr to strengthen the crypto on their site...

i2p2www/pages/site/get-involved/donate.html

@@ -23,10 +23,10 @@ For other payment methods, follow up with an email to eche|on
 with the subject "stickers please", your payment information, and your address.
 {%- endtrans %}</p>
 
-<h2><a href="http://bitcoin.org/">Bitcoin</a></h2>
+<h2><a href="https://bitcoin.org/">Bitcoin</a></h2>
 <p>{% trans date='December 2010',
 cointype='Bitcoin',
-coinurl='http://bitcoin.org',
+coinurl='https://bitcoin.org',
 account='1HkJCceXf7of1sTNRVJbXiZHfDTLL71Siy' -%}
 As of {{ date }}, eche|on has been running a
 <a href="{{ coinurl }}">{{ cointype }}</a> account for the I2P project.
@@ -55,7 +55,7 @@ mentioned on the I2P webpage.
 <img src="/_static/images/i2pltcdonate.png" alt="LTC donation QR code" /> 
 
 <br />
-<h2><a href="http://www.paypal.com/">PayPal</a></h2>
+<h2><a href="https://www.paypal.com/">PayPal</a></h2>
 <br />
 {% trans account='echelon@i2pmail.org' -%}
 You can donate direct via PayPal to the account "{{ account }}".
@@ -130,7 +130,7 @@ You can donate direct via PayPal to the account "{{ account }}".
 <br />
 <h2><a href="https://flattr.com/">Flattr</a></h2>
 <a href="http://flattr.com/thing/13523/Invisible-Internet-Project-I2P">
-<img src="http://api.flattr.com/button/button-static-50x60.png" title="{{ _('Flattr this') }}" style="border:0;" alt="{{ _('Flattr this') }}" /></a>
+<img src="/_static/images/flattr-button-static-50x60.png" title="{{ _('Flattr this') }}" style="border:0;" alt="{{ _('Flattr this') }}" /></a>
 <br />
 <br />