Add maintenance mode

This takes care of #579 

It adds the redis "maintenance" key as in american gut, as well as click commands to set and unset the maintenance mode. It also auto-sets maintenance mode if login screws up because the database is down.
Thanks!

On (Nov-21-14|11:50), Joshua Shorenstein wrote:
> > +# #############################################################################
> > +
> > +
> > +@maintenance.command()
> > +@click.option('--time', required=True, type=int,
> > +              help='The amount of time to lock the site, in seconds')
> > +@click.option('--message', required=True, type=str,
> > +              help="Message to show users")
> > +def lock(message, time):
> > +    r_server.setex('maintenance', message, time)
> > +
> > +
> > +@maintenance.command()
> > +def unlock():
> > +    r_server.delete('maintenance')
> > +
> 
> Yeah, I can add that
> 
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> https://github.com/biocore/qiita/pull/666/files#r20737504