allow modify max_per_page

[Fivell]

this small change just allow to change max_per_page in before_filter
(instead of #2810)

[seanlinsley]

I'd rather see us completely get rid of the static number in our code, making it fully customizable:

@max_per_page || active_admin_config.max_per_page

[Fivell]

@seanlinsley , I like idea of making max_per_page behaviour the same as per_page
I'll try to implement this when I'll have some free time.

[seanlinsley]

Closing as stale.