Create partials for every section of admin layout

Help users and extensions to customize those sections more easily
Remove useless modules from core spec_helper. [Fixes #3411]
Sets stock locations backorderable_default to false and stock item backorderable to false. Fixes #3300
Resolves issue #3401, improve readme on creating database for pre exising spree store

https://github.com/spree/spree/issues/3401