Don't embed f-droid logo SVG in each page. Reference from <img> instead.

Merged username-removed-25042 requested to merge pserwylo/fdroid-website:reduce-size into master

The .svg is 23KiB large, which adds quite a lot once you add up how many pages are output for our Jekyll site. Each of the 2000 package pages is only about 50KiB at most, so adding 23KiB to > 2000 .html files results in almost 100MiB of space, just for the header image.

By moving from including the svg source via Jekyll to using an tag, we save about 100MiB for free, and the browser will still cache the .svg just fine.