Allow determinism checking for entire jobsets

Setting

  xxx-jobset-repeats = patchelf:master:2

will cause Hydra to perform every build step in the specified jobset 2
additional times (i.e. 3 times in total). Non-determinism is not fatal
unless the derivation has the attribute "isDeterministic = true"; we
just note the lack of determinism in the Hydra database. This will
allow us to get stats about the (lack of) reproducibility of all of
Nixpkgs.

src/root/static/css/hydra.css

@@ -132,3 +132,13 @@ div.flot-tooltip {
     opacity: 0.80;
     z-index: 100;
 }
+
+td.step-status span.error {
+    color: red;
+    font-weight: bold;
+}
+
+td.step-status span.warn {
+    color: #aaaa00;
+    font-weight: bold;
+}