Docs: Add note on unsupported Django admin
Docs: Add notes on migrations with a custom User model

All credit for those findings goes to Dawid Lorenz.
Add docstring to dashboard's IndexView

Without it, Sphinx' autodoc for oscar.apps.dasboard.views would not
list the view nor it's members.
Docs: Cross reference dashboard navigation how-to.
Docs: Fix several syntax errors

* Use correct directives in signals.rst
* Removed newline in glossary.rst that broke sorting of glossary terms
* Removed syntax warning for upgrading.rst