css: mobile: do not hide sidebar

This is a quick fix to prevent the sidebar from disappearing on mobile
(<768px) displays. The better fix would be to show a different kind of
sidebar entirely for mobile devices, but this path is not worth
traveling at this time.
journals_controller: shorten long lines

No functional change.
use carrierwave instead of paperclip
journals: avatar image upload
journal: remove redundant code

Even without this, when we now save our new journal via the web
interface, we get a 'nil' type for the 'avatar' field.

Actually, we ran into a strange bug when this code was in place, where
the 'avatar' field would be assigned the value '_old_' by
carrierwave-mongoid. Removing the code fixes this.
journal: set avatar_type

The "#file#content_type" method chain already gives us the content type
(MIME type) for the "<journal_instance>.avatar" object. However, it's
still nice to have this content type information available to us as just
"avatar_type", for simplicity and also for agreeing with the information
already stored in BINARY_TYPES_HASH.