Refactor: Modularized footer.njk and header-footer.css

- Split header-footer.css into:
    - header.css
    - footer.css
    - modals.css
- Split footer.njk into:
    - footer.njk
    - modals.njk
- Updated front matter of:
    - index.njk
    - about.njk
    - credits.njk
    - gallery.njk
    - community.njk

src/gallery.njk

@@ -6,8 +6,10 @@ title: "Gallery - Bloom Valley Nursery Demo"
 stylesheet: 
   - "/styles/variables.css"
   - "/styles/base.css"
-  - "/styles/header-footer.css"
+  - "/styles/header.css"
   - "/styles/gallery.css"
+  - "/styles/footer.css"
+  - "/styles/modals.css"
 fontAwesome: "https://kit.fontawesome.com/c42448086d.js"
 currentPage: "gallery"
 pageScripts: