Add admin UI for shipping methods - stock loc association

Introduce the admin UI to be able to associate a shipping method to
specific stock locations.
This makes editable also `Spree::ShippingMethod#available_to_all` from
admin, being directly related to stock location association behaviour.

backend/app/views/spree/admin/shipping_methods/_form.html.erb

@@ -54,6 +54,32 @@
       <%= error_message_on :shipping_method, :tracking_url %>
     <% end %>
   </div>
+
+  <div class="col-5">
+    <%= f.field_container :stock_locations do %>
+      <%= f.label :stock_locations %>
+      <%= f.collection_select :stock_location_ids,
+        Spree::StockLocation.order_default, :id, :name,
+        {},
+        class: 'fullwidth select2',
+        multiple: true %>
+      <%= error_message_on :shipping_method, :stock_locations %>
+    <% end %>
+  </div>
+
+  <div class="col-5">
+    <%= f.field_container :available_to_all, class: %w(checkbox) do %>
+      <label>
+        <%= f.check_box(:available_to_all) %>
+        <%= Spree::ShippingMethod.human_attribute_name :available_to_all %>
+        <span class="hint-tooltip"
+              data-content="<%= t('spree.admin.shipping_methods.form.available_to_all_hint') %>">
+          <i class="fa fa-info-circle"></i>
+        </span>
+      </label>
+      <%= error_message_on :shipping_method, :available_to_all %>
+    <% end %>
+  </div>
 </div>
 
 <%= f.field_container :available_to_users, class: %w(checkbox) do %>

core/config/locales/en.yml

@@ -319,6 +319,8 @@ en:
         name: Name
         service_level: Service Level
         tracking_url: Tracking URL
+        stock_locations: Stock Locations
+        available_to_all: Available to all stock locations
       spree/shipping_rate:
         amount: Amount
         label: Label
@@ -883,6 +885,11 @@ en:
         expired: Expired
         inactive: Inactive
         not_started: Not started
+      shipping_methods:
+        form:
+          available_to_all_hint: When checked, this shipping method will be
+            available to all stock locations ignoring the selected values in
+            "Stock Locations" field.
       stock_locations:
         form:
           address: Address