Add groups to sort entries

[Fenykepy]

Add a way to group entries by groups:

• Groups are listed under main sections in left panel, separated from them by a rule;
• Add a button to create new groups separated from the new entry one (maybe in left panel);
• Add a multi-choices select in entry edition form to add entries to groups
• Add new route for groups: /groups/:group-slug/;
• Add an entry to a group by drag & drop from list to left panel.