Fixed list issues not loading with spaces in filtered values

Phil Hughes requested to merge issue-boards-filter-label-spaces into master

What does this MR do?

Vue resource is very kindly encoding our URL params when sending. However - we didn't actually want the + to be encoded because Rails reads that as a space, but doesn't read the encoded value as a space 😕

This converts the + into spaces 👍

What are the relevant issue numbers?

Closes #21920 (closed)

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