Improve Admin

[sveetch]

• [x] List field displays;
• [x] Useful list filters;
• [x] Fieldset structure to group fields by theme (translation? parameters? status? content? relations? dates? etc);
• [x] How to preselect language ? Enforce a select input in form with choices builded from language setting;
• [x] Enforce FK and M2M choice limits (original language, related not self, categories languages);
• [x] Test coverage;