Adds Poster uploader and new attribute in Gig

[#50719189]
Gig poster upload is enabled via form - Poster displayed on show and specific page

[#50719189]
Adds Poster to Seed

[#50719189]
Gig page displays acts thumb image - Band page displays gigs thumb image - CSS done

[#50719189]
Show venue displays gigs thumbs - Specific pages added for all gigs in venue

[#50719189]
Adds after destroy in gig for upload folder - Modified seed - Fixes bug for feedback partial when not signed in

[#50719189]
Removes authentication for gigs page in venues controller - Adds link to venue on gig page

[#50719189]
Merge pull request #17 from WilfriedDeluche/gig-poster-50719189

[Fixes #50719189] Gig poster 50719189
Resolves conflicts