Multiple value selection

[jimsafley]

There's interest in multiple value selection for the "Select menu" and "Custom vocab" input types. The best way forward is probably adding two new input types: "Select menu (multiple)" and "Custom vocab (multiple)" and rendering them as <select multiple>. That way there's no need for changes to the data model.

[jlayt]

I needed this, so decided to have a crack at it myself, my initial version is at molarchaeology@aef4b5d. It's not fully tested or polished, but seems to work OK for Items, Selects, and Custom Vocabs.

I took a different approach to Jim's proposal, rather than having separate Input Types, I found it easier to add a new multiple boolean field to collecting_prompts that each prompt type can enable and use if it can support multiples. Basically much of the code just duplicates the required field. For now I've just enabled it for the select-based inputs, i.e. Item, Select, and CustomVocab, but there's nothing to stop other prompt types adding support later if they make sense. I feel it reduces a lot of potential code duplication that might have existed with say two types for custom-vocab and custom-vocab-multiple.

I'm also thinking about adding a 'Display Option' to allow choice of tickboxes or radio buttons, but not fully investigated that as yet.