Client-side HTML 5 validations

[andrewhwaller]

Spotted an issue for this (#46) and figured I'd take a crack at it!

This PR adds a Requirable concern that checks a form's underlying class for a presence validator on the relevant field (presence_validated?) and merges required: true into the field's attributes if such a validation is detected. If required: false is passed in, that value will be respected. I added the concern to Checkbox, Input, Select, and Textarea, since those are the field types that can take advantage of required. I also added some specs covering the Requirable behavior for all four.

Note: I wasn't sure if a concern was the way to go here. Putting this logic directly in Field felt possibly weird, so I just went with a concern to keep things modular/contained.

[bradgessler]

Thanks for putting this together, @andrewhwaller! This was the inspiration for #70, which takes the same idea further — it handles required, minlength/maxlength, min/max/step, and infers them from length and numericality validators in addition to presence.

The main architectural difference is that #70 puts the validation inference on the Field (via a single prepend) rather than on each component individually, so everything flows through one place.

I'm going to close this in favor of #70, though I should note that #70 is a draft and I'm going to let it sit for a while so I can think about the approach. No immediate plans to merge. Thanks again for kicking this off!

[andrewhwaller]

No problem, glad I could help!