[Submissions] Improve clarity on pdf location on submisison page

In a submission page (/submissions/[preprint_id]/) the location of the paper's pdf is not easy to find, especially when the pdf is hosted in SciPost's preprint server (see first screenshot). The link to the preprint server does not even say pdf, and many people don't realise it is supposed to contain the paper itself.

In contrast, a published paper's hosting page (e.g., https://scipost.org/SciPostPhys.14.6.152) prominently displays the link to the pdf in a large boldface font (see second screenshot).

The submission page should change to more prominently display the link towards the pdf file, ideally in the same way the publication page does.