Merge pull request #30 from patfrat/master

Custom message to share links and more infos
Improvements on contributed enhancements for sharing files
Fixed shared links index view
Made it so that share_links.user_id is never empty. Improved index view for share_links.
Added @patfrat to list of contributors
Fixed bug in migration
Changed size of textareas
Improved to use where.not syntax
Better name for css class
Only execute update if there are active users
Added label to show that a message is optional