getDrops: Filter by channel implemented

* List of channel ids can be specified to filter the drop list.
Merge branch 'master' into rivers-dev
Added update SQLs.
getDropsByState Implemented

* Filter drops by read/unread state
getDropsByChannel implemented

* Filter drops by channel name.
getDropsByDate implemented
Added read status to Drop

* Set to true if the authenticated user has marked the drop as read.
searchAccounts Implemented

* Search Account path, name and email using the given query string.
Added avatar to account

* So we do not have to expose email any more.
River Collaborations implemented

* GET,POST,PUT & DELETE methods implemented.
Limit account search results.
Added bucket_name_canonical for buckets
Bucket collaboration implemented.
Added missing rivers table update sql.
Merge branch 'rivers-dev'