Fix the admin/content/signup administration overview page to better handle signup limits and not suck so much

The final piece of the signup limits design spec I wrote (and posted over at #359412-3: Move code for signup limits per status per node into a submodule) which needs further work is to somehow revamp the admin/content/signup administration overview page. However, this page is plagued with problems:

A) It's all hard-coded and inflexible.

B) It's very difficult to alter from other modules, since there's no way to alter the query used to generate it.

C) It currently displays unpublished nodes: #547758: exclude unpublished nodes from admin signup list

D) The operation links are kind of stale.

...

Ideally, I'd just replace it with a view. ;) See #582986: Make views a required dependency and ditch non-views code?

I guess I could even rip out that particular page and provide a default view for it, without making views a dependency. That page isn't essential to the operation of the module, so it could be something that's value-added once you enable views. There'd also be a VBO-enabled version of the view that let you do stuff like open/close signups in bulk, modify the signup limit, etc.

There's a start of an exported view for this page over at #582986-2: Make views a required dependency and ditch non-views code? which we could build on for this.

The hardest thing to replicate would be the magic for displaying the signup start time for all nodes, regardless of which kind of date field or event.module date info is being used for the signup start time of each node. We can't actually sort by that because it spans multiple DB tables and columns. I'll have to check with merlinofchaos about if there's any reasonable way to replicate the existing automagic behavior...