Adds Act resource with many to many relation between Gig and Band entities

[#48647481]
Nested attributes for main_act and supporting acts in Gig form

[#48647481]
Replace checbkox for _destroy attributes with JS boostrap button as data-toggle

[#48647481]
Optimization and design

[#48647481]
Adds unit tests for Gig and Act

[#48647481]
Solves conflicts

[#48647481]
Merge pull request #15 from WilfriedDeluche/model-act-48647481

Model act 48647481