Ignore code in WebplayerTemplates folder

Unity doesn't compile any code from the `WebplayerTemplates` folder (see [this blog post](http://theantranch.com/blog/avoid-compilation-with-webplayer-templates/)). Make sure Rider works nicely with this:

- [ ] Does Unity exclude the folder when generating projects?
- [ ] If included, tell Rider/ReSharper to ignore the files - don't create PSI, no inspections, etc.