Merge branch 'issue-boards-filter-label-spaces' into 'master'

Fixed list issues not loading with spaces in filtered values

## 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

See merge request !6258