Add time input to conribution form

After adding it as a separate field, I figured out that we should use
the same picker, because otherwise we'd have to keep the date in sync
depending on the time zone. Zones are too messy in that regard, and the
date is already normalized to UTC before creating the contribution
record.

closes #141

app/styles/_forms.scss

@@ -7,6 +7,10 @@ section#add-proposal {
     p {
       margin-bottom: 1.5rem;
 
+      &.label {
+        margin-bottom: .5rem;
+      }
+
       &.actions {
         padding-top: 1.5rem;
         text-align: center;