Fix rendering of metrics with special characters

My main motivation here is to get metrics with brackets to work in order
to support "pytest" test names:

- test_foo.py::test_bar[1]
- test_foo.py::test_bar[2]

I couldn't find an "HTML escape"-style function that would generate
valid html `id` attribute names from random strings, so I went with a
hash digest instead.

src/root/job-metrics-tab.tt

@@ -18,8 +18,7 @@
 
     <h3>Metric: <a [% HTML.attributes(href => c.uri_for('/job' project.name jobset.name job 'metric' metric.name)) %]><tt>[%HTML.escape(metric.name)%]</tt></a></h3>
 
-    [% id = "metric-" _ metric.name;
-       id = id.replace('\.', '_');
+    [% id = metricDivId(metric.name);
        INCLUDE createChart dataUrl=c.uri_for('/job' project.name jobset.name job 'metric' metric.name); %]
 
   [% END %]