Added input validation to help with cutoff date usage (#80)

lib/pinchflat_web/controllers/sources/source_html/source_form.html.heex

@@ -62,6 +62,9 @@
     type="text"
     label="Download Cutoff Date"
     placeholder="YYYY-MM-DD"
+    maxlength="10"
+    pattern="((?:19|20)[0-9][0-9])-(0[1-9]|1[012])-(0[1-9]|[12][0-9]|3[01])"
+    title="YYYY-MM-DD"
     help="Only download media uploaded after this date. Leave blank to download all media. Must be in YYYY-MM-DD format"
   />