Replace outdated icons with analytics symbol (#156)

https://weekly.ci.jenkins.io/design-library/symbols/ shows the preferred
images for Jenkins icons.  The symbols scale better and look more
consistent with the rest of Jenkins.

I chose "analytics" from the available symbols listed at
https://github.com/jenkinsci/jenkins/tree/master/war/src/main/resources/images/symbols

I added the page-headline class to the h1 headings because that seems
to be a class that is used in other page headlines.

Testing done:

* Confirmed that images were displayed as expected in freestyle projects
  and in matrix projects

src/main/java/hudson/plugins/plot/MatrixPlotAction.java

@@ -66,7 +66,7 @@ public String getDisplayName() {
     }
 
     public String getIconFileName() {
-        return "graph.png";
+        return "symbol-analytics";
     }
 
     public String getUrlName() {

src/main/java/hudson/plugins/plot/PlotAction.java

@@ -39,7 +39,7 @@ public PlotAction(Job<?, ?> job, List<Plot> plots) {
 
     @Override
     public String getIconFileName() {
-        return "graph.png";
+        return "symbol-analytics";
     }
 
     @Override

src/main/resources/hudson/plugins/plot/MatrixPlotAction/index.jelly

@@ -9,7 +9,7 @@
   <l:layout title="${it.project.displayName} plots">
     <st:include it="${it.project}" page="sidepanel.jelly" />
     <l:main-panel>
-      <h1>${%Plot Groups}</h1>
+      <h1 class="page-headline">${%Plot Groups}</h1>
 
       <j:choose>
         <j:when test="${it.hasPlots()}">
@@ -18,7 +18,7 @@
           </p>
           <table>
             <j:forEach var="group" items="${it.originalGroups}">
-              <t:summary icon="icon-graph icon-md" href="${it.getUrlGroup(group)}/">
+              <t:summary icon="symbol-analytics icon-md" href="${it.getUrlGroup(group)}/">
                 ${group}
               </t:summary>
             </j:forEach>

src/main/resources/hudson/plugins/plot/PlotAction/index.jelly

@@ -9,7 +9,7 @@
   <l:layout title="${it.job.displayName} plots">
     <st:include it="${it.job}" page="sidepanel.jelly" />
     <l:main-panel>
-      <h1>${%Plot Groups}</h1>
+      <h1 class="page-headline">${%Plot Groups}</h1>
 
       <j:choose>
         <j:when test="${it.hasPlots()}">
@@ -18,7 +18,7 @@
           </p>
           <table>
             <j:forEach var="group" items="${it.originalGroups}">
-              <t:summary icon="icon-graph icon-md" href="${it.getUrlGroup(group)}/">
+              <t:summary icon="symbol-analytics icon-md" href="${it.getUrlGroup(group)}/">
                 ${group}
               </t:summary>
             </j:forEach>

src/main/resources/hudson/plugins/plot/PlotReport/index.jelly

@@ -68,7 +68,7 @@
   <l:layout title="${it.group}">
     <st:include it="${it.job}" page="sidepanel.jelly" optional="true" />
     <l:main-panel>
-      <h1><l:icon class="icon-graph icon-xlg"/>${it.group}</h1>
+      <h1 class="page-headline"><l:icon class="symbol-analytics icon-xlg"/> ${it.group}</h1>
       <div>
         <st:adjunct includes="hudson.plugins.plot.PlotReport.jump-to-plot" />
         ${%Jump to} <select class="plot-selector" name="choice">