Implement lazy loading with relative links to icons SVGs

[mondeja]

Fixes #161 and fixes #143

The implementation for lazy loading in #55 didn't taken into account that lazy loading is not supported by browsers if images are included as base64 data URLs.

This changes the loading by using icon SVG relative links. Now the page loads fast because icons are only loaded as they appear in the screen.

Before

After

[LitoMore]

cc @jorgeamadosoria The #159 needs to be updated as well.

[jorgeamadosoria]

Looks good to me.
I don't see a need for unit testing the loading of a file: it's just accessing an url in the filesystem, it's not really code. That strikes me as overkill.
Otherwise, fine. Merge it so I can replicate it on #159 as @LitoMore said.