Delete unused files, restructure assets and use scss

app/static/css/materialize/fixes.css

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+.parallax-container .parallax {
+  z-index: 0;
+}

app/static/css/materialize/sidenav_fixed.css

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+/*
+ * The sidenav-fixed class is used which causes the sidenav to be fixed and open
+ * on large screens and hides to the regular functionality on smaller screens.
+ * In order to prevent the sidenav to overlap the content, the content (in our
+ * case header, main and footer) gets an offset equal to the width of the
+ * sidenav.
+ */
+@media only screen and (min-width : 993px) {
+  header, main, footer {padding-left: 300px;}
+  .modal:not(.bottom-sheet) {left: 300px;}
+  .navbar-fixed > nav {width: calc(100% - 300px)}
+}

app/static/css/materialize/sticky_footer.css

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+/*
+ * Sticky Footer: https://materializecss.com/footer.html#sticky-footer
+ * A sticky footer always stays on the bottom of the page regardless of how
+ * little content is on the page. However, this footer will be pushed down if
+ * there is a lot of content, so it is different from a fixed footer.
+ *
+ * Note: This may cause issues in Internet Explorer which has weak support for
+ *       flexbox.
+ */
+body {
+  display: flex;
+  flex-direction: column;
+  min-height: 100vh;
+}
+
+main {
+  flex: 1 0 auto;
+}