Admin can Manage Coupons for Merchants

As an admin,

When I visit a merchant's dashboard,

I see the merchant's link to manage their coupons.

When I click this link I am taken to the coupon index for this merchant, and can perform all actions that a merchant can on their coupons.

All limitations remain in effect for admin users when managing coupons.

child of #195
depends on #202
depends on #196
depends on #198
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