Links for unknown platforms should add generated CSS class for styling

[bekopharm]

First link is for example a syndication link to a diaspora* instance:

This can't be styled and since no icon are added the links never show up.

Something similar is done for the rel="me" links where I could simply override missing social media icons via CSS:

[metbril]

Or it could add a generic icon like this one:

https://github.com/dshanske/syndication-links/blob/master/svgs/website.svg

[metbril]

I have an issue with syndicating to IndieNews. The generated link is there, but doesn't show in the UI since an icon is missing.

For example:
https://robertvanbregt.nl/2018/12/31/indieauth-plugin-voor-grav/

[dshanske]

I will see about fixing the icon.

[dshanske]

It must have not been pushed to the repo in error