Allow ApplicationController and helpers to reload in development environment

[jvaill]

This fixes #697.

The problem is that PageController and its ancestors (up until ::InheritedResources::Base) don't reload by default and thus don't end up inheriting from future changes made to ApplicationController. It turns out that autoloading a class with ActiveSupport::Autoload does nothing to hook into Rails' reloading mechanism.

This PR moves the controllers to app/controllers/ which allows Rails to take over its autoloading and its reloading.

A good way to test this is with @timoschilling's minimal test case:
https://github.com/activeadmin/activeadmin_helper_reload_bug

🎉

[seanlinsley]

It seems like there should be a way to allow these files to be reloaded without moving them out of the lib directory.

By the way, last year there was a PR that did something similar, that didn't end up being merged: #2906.

[jvaill]

Just quickly brainstorming, off the top of my head I can think of a few other feasible solutions:

1. Add the entire lib/ directory to ActiveSupport::Dependencies.autoload_paths and let Rails handle autoloading and reloading. This has the disadvantage of unloading all files inside of lib/ on each request.
2. Move the controllers to a lib/controllers/ directory and add that to ActiveSupport::Dependencies.autoload_paths.
3. See if we can use ActiveSupport::Dependencies.explicitly_unloadable_constants to get Rails to explicitly unload the controllers after each request.
4. ? 😄

If we'd rather not shuffle files around, then looking into the third option might be worth it.

[timoschilling]

I agree with @seanlinsley, so I'm closing this, but I will copy your ideas over to #697.

[javierjulio]

@jvaill thanks. Your changes have been reincorporated in #8180 for now. I've confirmed they fix the helper bug reproductions locally. Just encountering another issue I'd need help with. We'll see if we can finally resolve this. Thank you.