Added holder.js in show details page

Temporary solution to display a placeholder poster when there isn't
any available for a show
Fixed javascript error

The sortable setup code would return an error when not
on the show list page
Added a parameter to displayFlashes to enable/disable the close button of alerts
Added ability to choose a custom poster from TheTVDB for each show

Closes #1
Slight CSS tweak
Added translations for custom poster feature
Changed alt text for posters in dialog
Returning 304 Not Modified if thumbnail hasn't changed based on If Modified Since header