add "dp_mirgation.sql" script to make migration to new data model easier for the rest of the team
double images fix for theme page (same slider layout for 'theme' and 'story')
show default thumbnail for media content which have no thumbnail
make exhibit home page layout empty because this layout isn't for 'real' pages. Page with "DPLA Exhibit Homepage" layour is container of exhibit meta-information (thumbnail, thumbnail caption and exhibit description)
Exhibition Overview Page: use ExhibitHomepage for displaying thumbnail and thumbnail caption
Exhibition Overview Page: display themes thumbnail on exhibit overview page instead of default image
Exhibition Overview Page: display ExhibitHomepage description as exhibit description
Exhibition Overview Page: do not display pages with layout "Exhibit Homepage" on overview page
fix Undefined index exception
fix Undefined index exception
fix non closed tag on Exbit overview page; support legacy data to display exhibit description