Move documents partials to components

This way it'll be easier to change them.

Note that there were two `.document-link` elements which aren't part of
a `.documents` element. We're renaming the HTML class of the link in
investments because it didn't contain links to download documents and
are slightly duplicating the CSS in the poll answer documents in order
to keep the `word-wrap` property.

app/assets/stylesheets/documents/documents.scss

@@ -0,0 +1,37 @@
+.documents {
+
+  h2 {
+    font-size: rem-calc(24);
+
+    span {
+      color: #4f4f4f;
+      font-weight: normal;
+    }
+  }
+
+  ul li {
+    padding-top: $line-height / 2;
+
+    &:not(:first-child) {
+      border-top: 1px solid $highlight;
+    }
+  }
+
+  .document-link {
+    background: $highlight-soft;
+    border: 2px solid $highlight;
+    border-radius: rem-calc(5);
+    display: block;
+    margin: $line-height / 2 0;
+    padding: $line-height / 2;
+    position: relative;
+
+    p {
+      margin-bottom: 0;
+    }
+
+    a {
+      word-wrap: break-word;
+    }
+  }
+}