Fix download button styling in multiselection dialogue

[Saira-A]

This PR renames the "select" button in the download selection gallery dialogue back to "download", which was the original label, for clarity. It also fixes a styling issue where that text only showed while hovering over the button.
This requires a manifest with selectionEnabled, e.g https://gist.githubusercontent.com/Saira-A/988dcfde9afc2971c842ae0d3392a91d/raw/30a5ea0d3b7595e3cbb40f17503c3d3f21064513/gistfile1.json (no download service so the button won't actually work).
Others have had issues getting that manifest to work so I've also included screenshots below. The "download" and "select all" labels are hard-coded in so that also needs to be fixed so will do later if needed (looks like only the BL used this feature and we might not bring it back anyway).

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[demiankatz]

Thanks, @Saira-A, I was able to use your sample manifest successfully. Was download previously renamed to select and is now being renamed back? I just want to be sure I understand the context of these changes, since in theory the rename could impact people's customizations (though in practice, I think it's relatively unlikely that people have customized this particular thing).

See below for other questions/observations:

[Saira-A]

Thanks @demiankatz, I've made the changes you suggested. This is an old screenshot from when it was in use at the BL, with the button labelled download:

I didn't manage to find any other institutions using the feature but I think it could cause confusion having a button named "select" right next to one called "select all" and "Download" is the more logical option.

[demiankatz]

Thanks, @Saira-A, this looks reasonable to me now. I'm approving but not merging yet -- do we want to merge this for inclusion in 4.3.0? If so, feel free to hit merge and I'll make sure it gets incorporated; if not, let's leave this open until after that release process is wrapped up.