Edit character code for Entypo's cancel icon.

I don't know enough about Entypo's toolchain to know why this was
the right thing to do, but I arrived at this guess after looking
at https://github.com/danielbruce/entypo/blob/master/config.yml

Bleeding edge, suckers.
Allow buttons to set a class name.
Add a close button and position it.
Delete unused variable.
Parameterize button and controls for danger.
Dispatcher can handle actions with no callbacks.
Store firebase url in variable.
Remove manuscripts from store on event.
Add key property to manuscripts in list.

Used internally by React.  The React dev tools where
telling me that it should be there.  They're probably right.
use less indirection on button mixins
Merge branch 'master' into delete-manuscripts
Merge pull request #3 from Structural/delete-manuscripts

Delete manuscripts
Merge branch 'master' into code-mirror
Move componentDidMount up, for consistency.
Wire codemirror up to dispatcher.