ENH: remove rendering datetime with Doxygen into HTML footer

The date and time are not set by javascript when the document is
loaded. This removes nightly modification of all the HTML files, and
enable traditional static HTML cache to work, and enable the generated
HTML to be committed to git regularly without nightly modification.

Documentation/Doxygen/DoxygenFooter.html

@@ -21,7 +21,8 @@
   </small>
 </div>
 <address class="footer"><small>
-$generatedby &#160;<a href="http://www.doxygen.org/index.html">
+Generated on <span id="datetime">unknown</span> for $projectname by &#160;
+<a href="http://www.doxygen.org/index.html">
 <img class="footer" src="$relpath^doxygen.png" alt="doxygen"/>
 </a> $doxygenversion
 </small></address>

Documentation/Doxygen/DoxygenHeader.html

@@ -14,6 +14,7 @@
 $treeview
 $search
 $mathjax
+<script type="text/javascript" src="$relpath^build_text.js"></script>
 <link href="$relpath^$stylesheet" rel="stylesheet" type="text/css" />
 $extrastylesheet
 </head>

Utilities/Doxygen/CMakeLists.txt

@@ -140,6 +140,16 @@ if (ITK_BUILD_DOCUMENTATION)
   file(COPY workbox-cli-config.js serviceWorker.js.in
     DESTINATION ${ITK_BINARY_DIR}/Utilities/Doxygen/)
 
+  execute_process(COMMAND ${PYTHON_EXECUTABLE} "${CMAKE_CURRENT_LIST_DIR}/datetime.py"
+     RESULT_VARIABLE CMD_RESULT
+     OUTPUT_VARIABLE _DATETIME)
+
+   if (CMD_RESULT)
+     message(FATAL_ERROR "Datetime failed!")
+   endif()
+
+   configure_file(${CMAKE_CURRENT_LIST_DIR}/build_text.js.in
+     "${ITK_BINARY_DIR}/Utilities/Doxygen//html/build_text.js")
 
   find_package( LATEX )
 

Utilities/Doxygen/build_text.js.in

@@ -0,0 +1,3 @@
+$(function(){
+    document.getElementById("datetime").textContent = "@_DATETIME@"
+});

Utilities/Doxygen/datetime.py

@@ -0,0 +1,21 @@
+#!/usr/bin/env python
+#
+#  Copyright NumFOCUS
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#         http://www.apache.org/licenses/LICENSE-2.0.txt
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+
+# Writes the date and time in RFC 2822 format to stdout
+
+import  time
+import sys
+sys.stdout.write(time.strftime('%a, %d %b %Y %H:%M:%S +0000', time.gmtime()))