[SPARK-20630][WEB UI] Fixed column visibility in Executor Tab

## What changes were proposed in this pull request?

#14617 added new columns to the executor table causing the visibility checks for the logs and threadDump columns to toggle the wrong columns since they used hard-coded column numbers.

I've updated the checks to use column names instead of numbers so future updates don't accidentally break this again.

Note: This will also need to be back ported into 2.2 since #14617 was merged there

## How was this patch tested?

Manually tested

Author: Alex Bozarth <ajbozart@us.ibm.com>

Closes #17904 from ajbozarth/spark20630.

(cherry picked from commit ca4625e)
Signed-off-by: Sean Owen <sowen@cloudera.com>

core/src/main/resources/org/apache/spark/ui/static/executorspage.js

@@ -492,24 +492,20 @@ $(document).ready(function () {
                         {data: 'totalInputBytes', render: formatBytes},
                         {data: 'totalShuffleRead', render: formatBytes},
                         {data: 'totalShuffleWrite', render: formatBytes},
-                        {data: 'executorLogs', render: formatLogsCells},
+                        {name: 'executorLogsCol', data: 'executorLogs', render: formatLogsCells},
                         {
+                            name: 'threadDumpCol',
                             data: 'id', render: function (data, type) {
                                 return type === 'display' ? ("<a href='threadDump/?executorId=" + data + "'>Thread Dump</a>" ) : data;
                             }
                         }
                     ],
-                    "columnDefs": [
-                        {
-                            "targets": [ 16 ],
-                            "visible": getThreadDumpEnabled()
-                        }
-                    ],
                     "order": [[0, "asc"]]
                 };
 
                 var dt = $(selector).DataTable(conf);
-                dt.column(15).visible(logsExist(response));
+                dt.column('executorLogsCol:name').visible(logsExist(response));
+                dt.column('threadDumpCol:name').visible(getThreadDumpEnabled());
                 $('#active-executors [data-toggle="tooltip"]').tooltip();
 
                 var sumSelector = "#summary-execs-table";