Disable the debugtoolbar icon on successful request

This gets in the way of the sidebar. It is possible to move it to
another location using debugtoolbar.button_style but there is nowhere
useful to put it where it doesn't get in the way.

Instead it can simply be loaded manually by visiting:

http://localhost:5000/_debug_toolbar/
Add a debugging section to the README

This introduces the debugtoolbar.
Remove debugtoolbar as a production dependency

We now install it as an extra requirement.
Merge pull request #1294 from hypothesis/debugtoolbar

Improve debug toolbar integration