Fix links to frontpage of documentation for old versions

I forgot that old versions have strange names for their homepage, and
we only switched to index.html in 7.2.

pgweb/docs/views.py

@@ -90,6 +90,15 @@ class _VersionPdfWrapper(Version):
 		self.__version = version
 		self.a4pdf = self._find_pdf('A4')
 		self.uspdf = self._find_pdf('US')
+		# Some versions have, ahem, strange index filenames
+		if self.__version.tree < 6.4:
+			self.indexname = 'book01.htm'
+		elif self.__version.tree < 7.0:
+			self.indexname = 'postgres.htm'
+		elif self.__version.tree < 7.2:
+			self.indexname = 'postgres.html'
+		else:
+			self.indexname = 'index.html'
 	def __getattr__(self, name):
 		return getattr(self.__version, name)
 	def _find_pdf(self, pagetype):

templates/docs/archive.html

@@ -20,7 +20,7 @@ If you are still using any of these releases, you are strongly advised to upgrad
     <td class="colFirst">{{v.tree}}</td>
     <td class="colMid">
           <ul>
-              <li><a href="/docs/{{v.tree}}/static/index.html">Without user comments</a></li>
+              <li><a href="/docs/{{v.tree}}/static/{{v.indexname}}">Without user comments</a></li>
           </ul>
       </td>
       <td class="colLast">