Refactors Admin::PagesController#create for better readability

Same behavior as before, just code cleanup.
Updates to latest version of non-stupid-digest-assets

NOTE: Since the new version is released on rubygems, please make sure that you have the gem in your Gemfile:

gem 'non-stupid-digest-assets', github: 'alexspeller/non-stupid-digest-assets', branch: 'master'
Updates installation instructions for Alchemy v3.0 [ci skip]
Make the current_user configurable

Makes the function configurable that is called by `current_alchemy_user`
to get the current user. Set Alchemy.current_user_method to the name
of the custom function.