Update product prices when customer is changed (SHUUP-2719, SHUUP-2721)

[gurch101]

Fix line item pricing to reflect customer/campaign discounts for admin
order editor. Use product.get_price_info to get prices.

Refs SHUUP-2721, SHUUP-2719

[anders-jenkins]

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Test FAILed.

[coveralls]

Coverage decreased (-0.004%) to 86.24% when pulling 7ab85600a38b4a9a0ed11550e851b445cf74e2f5 on gurch101:admin-order-customer-refresh-fix into cfb3fa3 on shuup:master.

[anders-jenkins]

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Test PASSed.

[anders-jenkins]

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Test FAILed.

[coveralls]

Coverage remained the same at 86.244% when pulling 0a39dfd on gurch101:admin-order-customer-refresh-fix into cfb3fa3 on shuup:master.

[anders-jenkins]

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Test FAILed.

[tulimaki]

retest this please

[anders-jenkins]

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Test PASSed.

[suutari-ai]

Good work, @gurch101! Thanks!