Improve Show As Tabs by Default Class

[ElebrUS]

Problematics:

• Suppose I already have a finished project and I want to connect unfold admin to it.
• All my models are highly loaded and have a lot of fields that I want to group by tabs, specifying classes for each fieldset is a very labor-intensive task

Solution:

• Add default class settings for all fieldsets that can be overridden in one/few models

Features:

• Added defining a default name for a fieldset if it's a tab and it doesn't have a name assigned to it, i.e. None