New patch release: v5.2.1

This is a patch release to fix one significant bug: when additional shipping fee was set to 0, there was no shipping fee included.

Changes v5.2.1

[fix] fix: calculateShippingFee (when shippingPriceInSubunitsAdditionalItems is 0, no shipping fee was included) #414
[fix] Remove stock from schema if there's no stock in use. #405
[fix] Remove left-behind slash from inquiry-new-inquiry email template reference. #406
[fix] The subject line of purchase-new-order email had a wrong variable name. #413
[change] Fix another typo in FR translations. #409
[change] Fix a typo in FR translations. #408
[add] Add currently available translations for de, es, fr. #404
[fix] The example files of SignupForm and ConfirmSignupForm had wrong data. #403
[change] FilterComponent: relax generated name-attribute for inputs: allow camelCase. #402