enabled ActiveRecord based session store and make it configurable
default to cache store in production
remove unnecessary calls to presence
make session store configurable via env var SESSION_STORE
set default in configuration.rb
infer content type from contents of file instead of its file extension
* defaults to application/binary as its more secure than octet-stream
* set nosniff header to instruct browsers not to guess content type
migration to set content types of all existing attachments
Merge pull request #1239 from opf/fix/database_session_store

enabled ActiveRecord based session store
Merge pull request #1171 from opf/retrieve_content_type_from_file

infer content type from contents of file instead of its file extension
Merge branch 'release/3.0' into dev