Clean up checks to avoid db connection on precompile

We still need to deal with the Preference / Config issue as stated in #3833
Bump awesome_nested_set

Get fix to avoid touching the database on assets precompile
Accept source attributes in PaymentsController to fix backend payment processing.

Fixes #3823
[backend] Use the controller name rather than Object when a model does not exist for authorize_admin method

Fixes #3622
Move preference_resuce.rb into Core, where it should've been

Fixes #3860
[frontend] Removed scopes/dynamic.rb

The price_scopes_for method defined in this does not appear to be used anywhere
Add changelog entry for #3860
Fix issue where cancelling a line item delete didn't actually cancel it

Fixes #3862
Refactor ReturnAuthorization#generate_number

Fixes #3864
[backend] Added eager loading for variant + product to stock movements/index
[backend] Link to a shipment's order in stock_movements/index view
[api] Add eager loading for option values + option types to stock items
[api] include option_type presentation within option_value_attributes
[backend] More clearly differentiate select2 result elements
[backend] Split variants/_autocomplete into two templates which can be used independently
Do-over "Add Stock Movement" form

If there are a large number of stock items for this stock location, the page can take some seconds to load, as mentioned in this thread:

https://groups.google.com/forum/#!topic/spree-user/nu0_NlNGPV8

To make that stop being a problem, I've switched that UI to using select2 to load stock items 50 at a time. I've also imporved the UI to show more information about the product, such as its SKU and its options text.
[backend] Include options text when displaying variants on the stock movements page
Update changelogs for latest issues
Remove master branch mentions in the README
Tag 2.1.2