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/
Restore default view to 'Screen'

As a part of the fix for #1231 in commit
https://github.com/hypothesis/h/commit/f66448fc3b2d1e07cbb491db73367b4b69e5f9a6
the default view was change to 'Document' from 'Screen'
but the annotations were not loaded into the sidebar as supposed
for 'Document' view, because the applyView was called before
the annotations were loaded.

Since it'd take effort to correctly implement Document view update
when the store loads new annotations, it is feasible to return
to the original 'Screen' view, which works well, because
views are planned to be refined, maybe redesigned.

Fix #1276
Merge pull request #1295 from hypothesis/1276-load-annotations-to-sidebar

Restore default view to 'Screen'
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
Don't install dev dependencies in prod
Update CHANGES.txt
Add issue numbers to changelog
Remove unnecessary leading slash in asset base url
Start building up a pattern library

Starting with the form elements, the new view is available at
/dev/pattern-library and currently only shows form fields.
Move pattern library view to h.testing

This ensures that it's only loaded in development.
Add experimental form styles

These new styles include labels by default and error styles. It also
gives a little more breathing room between form elements.

The buttons now have a more consistent hover/active state without the
glossy highlight and the disabled state looks disabled.
Add forms.scss to the assets.yaml
Update the login form to trial new form styles
Use Compass for button gradients
Update registration forms to use new styles
Update the forgot password form to use new styles
Update the activation form to use new styles
Add new btn-clean style

This removes the background and borders from the button.