Support custom actions as a first-class entity

[jesseduffield]

Copied over from #222 (comment) (forgive me not using the specified template)

I've been taking a look at Administrate as an alternative to RailsAdmin and it looks like the ability to easily add custom actions is the main thing that's currently missing. In rails admin you can specify actions against models and those actions will show up next to the default routes like so:

You can implement each action either as immediately doing something upon clicking it, or having it first show a form and then doing something upon form submission.

Actions also automatically appear at the end of tables for easy access:

I think that actions should be considered a first-class entity in Administrate, just like they are in RailsAdmin, and I believe that Administrate can find a way to implement it that aligns with its value of using as much regular rails as possible.

I'm eager to switch over to Administrate because I much prefer its general approach to RailsAdmin but it sounds like this shortcoming is going to be too onerous to justify the switch.