Make supporting files option always visible but optional

[hdrury1]

Hypothesis: Making supporting files visible at all times with clear information of what they should be used for, and clearly show as optional may help reduce the amount of files uploaded "just in case" but we don't actually want.

This move would also allow us to get rid of the text above the cover letter editor asking users to add details of relevant published work and upload in supporting files (which currently isn't visible until manuscript is uploaded).

Successful outcomes: We see more uploaded "related studies" and fewer supporting files uploaded that we don't want e.g. cover letters