Choose contributors from list for new proposals

app/styles/components/_add-contributor.scss

@@ -15,7 +15,7 @@ section#add-contributor, section#add-proposal {
       }
     }
 
-    input[type=text] {
+    input[type=text], select {
       width: 100%;
       padding: 1rem;
       border: none;
@@ -26,6 +26,31 @@ section#add-contributor, section#add-proposal {
       &:focus, &.valid {
         background-color: rgba(22, 21, 40, 0.6);
       }
+      @include placeholder {
+        color: rgba(238, 238, 238, 0.5);
+      }
+    }
+
+    select {
+      -webkit-appearance: none;
+      -moz-appearance: none;
+      appearance: none;
+      border-radius: 0;
+      background-color: rgba(22, 21, 40, 0.6);
+      background-image:
+        linear-gradient(45deg, transparent 50%, gray 50%),
+        linear-gradient(135deg, gray 50%, transparent 50%);
+      background-position:
+        calc(100% - 1.5rem) calc(1rem + 0.5rem),
+        calc(100% - 1rem) calc(1rem + 0.5rem);
+      background-size:
+        0.5rem 0.5rem,
+        0.5rem 0.5rem;
+      background-repeat: no-repeat;
+
+      &:invalid {
+        color: rgba(238, 238, 238, 0.5);
+      }
     }
 
     input[type=submit] {