Add change tracking to forms and rewrite validation

[TolikPylypchuk]

Change tracking will enable the forms to be more straightforward. For example, changing the poster will be a separate API call, and as such it should be done on an unchanged form, otherwise cancelling the changes will not lead to cancelling the poster change, which may confuse users.

[TolikPylypchuk]

Validation should be rewritten as I don't really like the current approach - it was simple in the beginning, but now it's grown to be too complex. I already have a new way of doing it in mind - it will also allow me to seamlessly add change tracking to forms.