Add ability to pass params into forms

[xmorave2]

This enables to create links with urls like /Feedback/Form/SomeForm?param=value to prefill inputs for user, or fill hidden inputs with some dynamic value.

[demiankatz]

Thanks, @xmorave2! I'm curious whether @EreMaijala has any thoughts on this, since he's more invested in the form code than I am. I also wonder whether we need to add test coverage for this new functionality, and whether we should add a configuration setting to limit which parameters may be pre-filled via URL (in case there are any possible abuses of the functionality that we might want to avoid). Any thoughts?

[EreMaijala]

This is a good idea, but @demiankatz has valid points. We need to protect at least the read-only reserved elements (referrer, useragent, submit). Also need to ensure that this doesn't allow overriding a select field with an invalid value (I didn't test it yet), and here test coverage would help.

[xmorave2]

Thanks for review.

I added configuration for enabling prefill only for specific fields and check if some of protected fields are not prefilled.

I didn't implement prefilling for selects and checkboxes - it turned out a bit complicated and we don't even need for our use case.

[demiankatz]

Thanks for the progress on this! I have just a few minor suggestions.

[xmorave2]

Thanks @demiankatz I fixed them all.

[demiankatz]

Thanks, @xmorave2! I'll wait to see if @EreMaijala has any further thoughts before merging this.

[EreMaijala]

Looks good to me now!