These days, to override a mixin it is only required to override the
OrderPlacementMixin; overriding the app and view is not necessary. It is
just a generic case of overriding a core class. A dedicated how-to is
thus unnecessary.

As overriding a core class is the general goal of any advanced
customisation, it has been merged into the customisation section, which
has been given an overhaul as well.

The section regarding modifying apps will be re-added as part of a
higher-level "the Oscar application tree" kind of document.

It's getting easier and easier to customise parts of Oscar; the
customisation section can do with a rewrite because of it. Added to my
to-do list.

The customisation section required some changes as it's not often
necessary to override the root application instance to change URLs or
views of subapps.

* Better wordings for the individual steps
* Mentioned necessary copying of migrations directory
* Fixed wrong prerequisites in examples
* Glossary typo fixed

Attempt at making it clearer what needs to be done for what kind of
customisation. Created a new topic that lines out the prepatory
steps that need to be taken, and moved duplicate information from the
how-tos into the new topic.