Forms - Remember the last signer selection on a per-form basis for each user submitting it

Currently, if there are multiple form signer options and we are not able to determine the most appropriate choice, the options are just shown alphabetically. This causes forms to often be submitted with the wrong signer selected.

To mitigate this, remember the last signer that was selected for each form and load that as the default option the next time that the form is submitted.

We may also want to initially force the form submitter to choose the most appropriate option instead of just having the first option selected. This has pros and cons though, especially when it comes to bulk form submissions.