Add one-shot jobsets

There are jobsets that are evaluated only once, that is, after they've
been evaluated, they're disabled automatically.  This is primarily
useful for doing releases: for instance, doing an evaluation with
"officialRelease" set to "true" should be done only once.

src/root/jobset.tt

@@ -116,6 +116,10 @@
   <div id="tabs-configuration" class="tab-pane">
 
     <table class="info-table">
+      <tr>
+        <th>State:</th>
+        <td>[% IF jobset.enabled == 0; "Disabled"; ELSIF jobset.enabled == 1; "Enabled"; ELSIF jobset.enabled == 2; "One-shot"; END %]</td>
+      </tr>
       <tr>
         <th>Description:</th>
         <td>[% HTML.escape(jobset.description) %]</td>
@@ -127,10 +131,6 @@
           <tt>[% HTML.escape(jobset.nixexprinput) %]</tt>
         </td>
       </tr>
-      <tr>
-        <th>Enabled:</th>
-        <td>[% jobset.enabled ? "Yes" : "No" %]</td>
-      </tr>
       <tr>
         <th>Check interval:</th>
         <td>[% jobset.checkinterval || "<em>disabled</em>"  %]</td>