The `alt` attribute that was automatically generated as part of the
`{% image %}` tag could not be overridden. If template authors passed
their own `alt="..."` attribute in, two would be printed out instead
of the default one being overridden.
The relevant code has been refactored to build a dict of attributes,
allowing the default set to be overridden, and then printing them using
`flatatt`.
Fixes #1933
All `.. code::` instances have been changed to use `.. code-block::`,
and have been properly formatted. The syntax names have been normalised,
so all django templates use the `html+django` syntax, shell commands use
`sh`, and plain text uses `text`.
