Add F-Droid download button

Using the mechanism introduced for iOS to describe the multi-step
process.

Fixes #52.

snikket_web/scss/invite.scss

@@ -74,6 +74,10 @@ img.play {
 	height: $w-l3;
 }
 
+img.fdroid {
+	height: $w-l3;
+}
+
 .tabbox {
 	display: flex;
 	flex-direction: column;