Attempt to restore 2.x config loading behavior where 1) per-model config blocks
only evaluate after all the models are loaded and associations are defined
while 2) initializer config block evaluates immediately so that routes are
configured correctly.

One remaining change compared to 2.x is RailsAdmin::Config#model no longer
returns the model when given a block. With the removal of LazyModel in e4ae669
it is tricky to both defer initialization and return the model class.

Fixes railsadminteam#3490

In 3.x the initializer config blocks are intentionally deferred to allow
referencing model classes without quoting. Here is a first pass at keeping that
behavior on top of ec03eb6. It could probably use a little cleanup but here's
an idea of an approach for feedback.

See
railsadminteam#3492 (comment)
for details.

…config-loading-2