When multiple recipes with inputs are activated, show all their inputs in a single form

Problem/Motivation

If you activate multiple recipes that have inputs, it...doesn't work. You'll be redirected to an input form for the first one, but not the others.

Proposed resolution

TBD, but probably we should redirect to a single form that knows which recipes you're trying to activate, and thus collects inputs for, and applies, all of them at once.

This will probably necessitate some backwards compatible changes in the activator system. We need some way for activators to indicate that they will process multiple projects at once. I discovered that we can keep ActivatorInterface as-is and do this in implementations:

public function activate(Project ...$projects): ?array { ... }

This would be an entirely reasonable way for an activator to show that it will handle, in one call, every project that it supports. Both ModuleActivator and RecipeActivator could implement this. This would allow RecipeActivator to redirect to RecipeForm once and handle several recipes.