Split pickup-related gateway functionality into Store\PickupGateway

What does this MR do?

Split pickup-related gateway functionality into a new Store\PickupGateway.

The only shared data query was getStoreName, which is not complex.

How confident are you it won't break things if deployed?

If the backend tests run successfully, everything should still work in theory, since code was only moved to different places.

Links to related issues

• preparation for #1029 (closed)
• !1800 (merged) <- you are here
• !1809 (merged)
• !1810 (closed)
• !1811 (merged)
• !1814 (merged)

How to test

basically do anything that relates to the moved store gateway parts.

Checklist

• only moved around backend parts
• no unrelated changes
• asked someone for a code review
• set a "for:" label to indicate who will be affected by this change
• use "state:" labels to track this MR's state until it was beta tested
• added an entry to CHANGELOG.md
• no release notes forDev
• no beta testing (should happen combined with the big StoreModel changes)