WIP: pivot table

[dedobbin]

Work in progress. Some things that need to be done from top of my head:

• FE should check model exists.
• When pivot model fields are left empty, should use default.
• BelongsToMany relations are now 'pseudo-relations' in backend (A related to AB, AB related to A).
  Maybe would be better if these 'pseudo-relations' exist in frontend only, because they are only there for visuals, and aren't actual model relations. They are not stored though (outside of cache) so maybe it's fine this way.
• Migrations for relations when generating pivot model with relations.
• General clean, not all parts are 'equally elegant'. Also some TODOs can be found in the wild
• I think belongsToManyMigrations dont delete correctly when the checkbox is used