Update Tidelift docs with latest campaign

Tidelift has launched a new marketing campaign as outlined here:

* https://forum.tidelift.com/t/task-enhancement-marketing-the-tidelift-subscription-to-your-users/321

This PR splits the previous "sponsor" information into two, Open Collective
and Tidelift (as they have very different target audiences).

Also took the opportunity to reorder some items at the end of
the contents page in a manner that I believe make more sense.

doc/en/_templates/globaltoc.html

@@ -12,6 +12,7 @@
   <li><a href="{{ pathto('backwards-compatibility') }}">Backwards Compatibility</a></li>
   <li><a href="{{ pathto('py27-py34-deprecation') }}">Python 2.7 and 3.4 Support</a></li>
   <li><a href="{{ pathto('sponsor') }}">Sponsor</a></li>
+  <li><a href="{{ pathto('tidelift') }}">pytest for Enterprise</a></li>
   <li><a href="{{ pathto('license') }}">License</a></li>
   <li><a href="{{ pathto('contact') }}">Contact Channels</a></li>
 </ul>