Improve form autosave

[EdoStorm96]

As it stands, forms basically autosave on every keystroke (or every keystroke per N milliseconds) but we'll need to move to a somewhat smarter system.

For a start, I think it would be a good idea to submit on:

• clicking "Next" ot "Previous" to a new step.
• Answering a question which can trigger a condition.

We can either save on blur for Question types which can potentially have conditions (probably mostly select's, radio's and multiple choice field, maybe date fields) or we could specifically check if a question has conditions connected. To me the latter option seems preferable, but @XanderVertegaal has volunteered to pick this up and can decide what is better.