Sections and fields: label actions correctly

Currently, the pages field and sections both feature an Add button although the one lets you select pages while the other lets you create new pages. While this might only sound like a minor difference when used separately, the separation is important for editors to understand the different options when both – pages fields and sections – are used in conjunction. With the identical labeling, a pages field can be mistaken for a section and editors can be confused why there is no create option.

The labeling should be consistent across the different sections and fields (files, pages, users). There was a similar request for files before: https://kirby.nolt.io/167