feat: make stepper bubbles collapse and refine css working

[EdoStorm96]

So, I was just puzzling a bit with the stepper, and to my surprise, came up with a solution for making the bubbles collapse ...

This is still a bit rough, but I thought I'd share it with you before the (my) weekend. Will add more notes on monday!

[EdoStorm96]

So here is the new version of this PR, some notes on what has been changed:

• StepperItem's now have an is_available attribute. This is false by default. This relates to whether its children are displayed. A more correct name would beis_not_collapsed` ...
• The new way of searching for the current item is retained, but this now happens in a separate method (item_is_current_check()) of the stepper, until a current item is found.
• When a current item is found, its get_ancestors() gets called, which returns a list of the item + all of its ancestors (so parent's, parent.parent's etc) These items' is_available are then set to True
• The get_errors function and get_css_classed have had a bit of an overhaul. get_errors, can now get an include_children boolean argument, so that the errors of an item's descendants are also taken into account.
• The current design is, when an item is collapsed, its completeness gets evaluated with include_children, when an item is_availabe just the model_form errors and the optional get_checker_errors get evaluated.

This feels right to me :)

[EdoStorm96]

So, I've adressed your comments. Good ideas!

I also noticed that FundingForm did not really include validation, so I just added some cuz it was bugging me ...