[TASK] Deprecating v:if.client.*

User agent sniffing is bad practice. We shouldn't support it. It also doesn't make sense in a template context, as most users will have it cached, which leads to unwanted behavior that we have to deal with on support.

> [13:10:44] <@cedricziel>	 thats bad practice and i think we should educate 1st: why it is 2nd: that the core does this, if you want conditional stylesheets etc
[13:10:59] <@cedricziel>	 1+
[13:11:06] <@cedricziel>	 one less paintpoint