View for open slots

This is mostly supported in 3.x, but needs the following enhancements:

* Set and forget mode handles capacity being reached, not just open/close dates
* A new Views field should be added for Register link, with checkbox to allow showing registration status instead if registration is closed

If this is mostly supported, it would be good to document this feature. For now I can see ho I can add available spaces as a field, but the filter still seems to be not implemented. Should I create a new issue for this?

Thanks @milos, let's keep things in this issue - this can pop to the top of my to-do list. I'll get the filter etc into the next release, I'll use this issue. And yes - fresh documentation is needed for all features of the module - that is coming over the next few months.

Added new filters for:

* Host entity spaces remaining
* Host entity spaces reserved
* Host entity registration count

Committed to dev branch and will be in the next release.