Twenty Fifteen: Social Links Menu and Skype URIs

[afercia]

In the main stylesheet there's one rule to support Skype links in the Social Menu, btw they won't work unless you add a "skype" protocol in the kses allowed protocols. Without that, you can't save a link with a "URL" value like, for example, "skype:echo123?call".

Proposed patch uses the "kses_allowed_protocols" filter and also fixes the doubled ":before:before" in the stylesheet.

[iamtakashi]

I'm kind of ok to remove Skype support rather than adding a Skype protocol.

[afercia]

no objections to remove it, if there are security concerns, otherwise I don't see so much difference between "skype:" and "mailto:" links (which are supported)

[iamtakashi]

Replying to afercia:

no objections to remove it, if there are security concerns, otherwise I don't see so much difference between "skype:" and "mailto:" links (which are supported)

OK, let's remove Skype. Do you want to cook a quick patch?

[afercia]

Oh I guess there's no need, it's faster and simpler if you remove the rule from style.css I can live without a prop :)

[iamtakashi]

Remove Skype from supported social links since we need to add their protocol.

[iandstewart]

In 30010:

Twenty Fifteen: removing Skype from the list of social links is easier than adding skype to $allowed_protocols.

Props iamtakashi, fixes #30059.