A couple perf fixes for /admin homepage

I noticed that /admin was quite slow (~9 seconds) on prod and *really* slow in development (~45 seconds!).

I think two things have happened:

1. We have a lot more orders now
2. We have a *lot* more promos as of today (we went from under 3,000 to over 50,000 total today)

Regarding item 1 - for whatever reason postgres really doesn't like "distinct" modifier when querying orders like this.

Before the changes to orders_controller:

    Spree::Order Load (2190.6ms)  SELECT DISTINCT "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."deleted_at" IS NULL AND ('t'='t') AND ("spree_orders"."completed_at" IS NOT NULL) ORDER BY "spree_orders"."completed_at" DESC LIMIT 15 OFFSET 0

After:

    Spree::Order Load (   1.4ms)  SELECT          "spree_orders".* FROM "spree_orders" WHERE "spree_orders"."deleted_at" IS NULL AND ('t'='t') AND ("spree_orders"."completed_at" IS NOT NULL) ORDER BY "spree_orders"."completed_at" DESC LIMIT 15 OFFSET 0

Regarding item 2 - the current code for the Orders#index page queries *all* promotions (i.e. `SELECT "spree_promotions".* FROM "spree_promotions"`) and loads all 50,000 into ActiveRecord objects and then renders them into 50,000 `<option>` tags in the html page.

HMTL page size before update to orders/index.html: 5.4 MB (_mega_)
HTML page size after: 37.1 KB (_kilo_)

Total diff for both changes in development mode:

Before:

    Completed 200 OK in 47340ms (Views: 43722.9ms | ActiveRecord: 3370.1ms)
    47.62s (browser)
    5.4 MB page size

After:

    Completed 200 OK in 1769ms (Views: 1374.2ms | ActiveRecord: 337.8ms)
    1.87s (browser)
    37 KB page size