Masonry fix.

* Removed margin around drops causing them to span multiple rows.
* Reduced the drop's width slightly.
Drops View: Display drop image
Display river and bucket lists in the header modal views.
Basic display of drops in river.

* Pending infinite scrolling and polling.
Create a river

* Shows the create river form in the header modal.
* Redirects user to the river when successfully created
UI: System messages, Stream messages and button progress indicator
Merge branch 'ui-dev' into rivers-ui-dev
Display channels modal.

* Display list of channels in a river when channel settings clicked in
drops listing.
Add Channels

* Add channels modal implemented.
* Pending opml import, busy and error messages.
Delete channel from add channels modal.

Clicking on the 'X' removes the channel.
Modify Channel

* Editing a channel from the channels modal implemented.
Display channel list on river drop listing

* Filtering the drop list depending on the selected channels is pending
Added static river settings pages.
River Display Settings

* Allow update of river name, privacy and description.
* System message ui added to signal success/failure
Error Handling

* Throw different types of expceptions per api error.
Infinite scrolling

* Returned page parameter
* Backbone fixes. add parameter is now update during fetch + issue with
comparator for drop list sorting.
Login: Display errors as system messages.
New drop polling

* get_drops_since implemented in river service
* seems there is a change with how Backbone iterates on models added to
a collection for the add event. Models are iterated over in the order
they are added and not their natural order. Reversing the droplist in
Controller_River::action_droplets to resolve this for now.
Logout on authorization exception.