Implement UniFormItem helper

[warpech]

Implement UniFormItem helper for the custom element <uni-form-item> (development of which is tracked in https://github.com/Starcounter/RebelsLounge/issues/261)

UniFormItem can be used for:

• read-only rendering of a single view-model property with a label, value, and a message
• editable rendering of a single view-model property with a label, value, validation result and output (that can be a neutral message, positive validation result, negative validation result)

The view-model property can be bound to database object property.

cc @joozek78

[warpech]

This is being implemented in https://github.com/Starcounter/Starcounter.Uniform/pull/6

[PatrykSzwer]

Done in #6.