Split up contributing guidelines

They were getting quite long. They've been split up in several
subsections. Some of them are borrowing heavily from the Django docs.
Docs: Merged content from old docs

my_model_customisations_arent_picked_up now lives in a subsection of the
recipe how_to_customise_models.

The old conventions.rst is now merged into the new coding-style.rst.

The old search.rst has been dropped as it seems outdated.
Docs: Centralise sandbox instructions

Building the sandbox was explained in multiple locations, and in
slightly different ways. The sandbox now has a separate document.
Docs: Add debugging trick for model customisations

Was posted on the mailing list.
Docs: Added documentation for tox tests
Docs: Merge tests/README.rst into docs structure
Docs: Move design decisions and dev environment

They really don't belong in /contributing.
Remove obsolete 'form-actions' section from underneath the page edit form
Merge pull request #603 from zoyalab/pages-dashboard-fix

Removed obsolete 'form-actions' section from the page edit form
Remove django from requirements.txt

This was causing tox to choke as there were conflicting requirements
(well, kind of).  Removing it allows tox to run normally but means we
can't run 'upgrade' without conflict.

I've adjusted the makefile to use install for the 'ci' target now
instead of upgrade.

Fixes #600
Add profiling decorator

...and repackage profiling middleware.
Default to having debug toolbar disabled

Makes sandbox MUCH faster.
Use cached version of categories on listing view
A few minor corrections to the docs
Include snippet for better documenting of model fields

See http://djangosnippets.org/snippets/2533/
Add simple address app docs (including autodoc)
Tests: Check new password works after changing it.

The profile test can_change_password checked if the password change
form can be submitted succesfully, but did not verify that Django's
authentication system accepts the new password.
Merge pull request #580 from tangentlabs/issue/580/docs

Docs: Contributing guidelines and legacy docs