[No ticket] Preprint branding rework (#1913)

-   Ticket: []
-   Feature flag: n/a

- Only rely on `brand` relationship for setting preprint colors

- Use `brand.primaryColor` for branded navbar background color
  - Add styling if the brand's primaryColor does not provide sufficient contrast with white text
 - Add special-case for BioHackrXiv to change navbar color to white (their primary color would be white, but that creates problems for `<Button>` behavior, so their primary is black instead)
- Use `brand.heroBackgroundImage` for hero banner background, as well as advisory board section when @brianpilati implements that page. Currently just a solid color
- Use `brand.navbarLogoImage` for navbar logo if `preprintProvider.assets.square_color_transparent` is not defined
- Update `<Button>` component to detect primary color and use white/black text depending on color contrast

app/preprints/template.hbs

@@ -1,8 +1,10 @@
 {{page-title this.theme.provider.providerTitle replace=true}}
-<ThemeStyles />
-<BrandedNavbar
-    @translateKey='collections.general.brand'
-    @brandRoute='preprints.discover'
-    @addLinkKey='collections.navbar.add'
-/>
-{{outlet}}
+<div {{with-branding (get-model this.theme.provider.brand)}}>
+    <ThemeStyles />
+    <BrandedNavbar
+        @translateKey='collections.general.brand'
+        @brandRoute='preprints.discover'
+        @addLinkKey='collections.navbar.add'
+    />
+    {{outlet}}
+</div>