Support form attribute on input elements

[tjwallace]

HTML5 allows inputs to be explicitly associated to one or more forms via a form attribute.

I think this would require updating the ng-model directive to look for forms by ID if the form attribute is present on the input, otherwise follow the current logic.

This would help get around the nested form issue as well, #5858.

Example:

<form id="form1">
  <input type="text" form="form2">
</form>
<form id="form2">
  <input type="text" form="form1">
</form>

<input type="text" form="form1">
<input type="text" form="form2">

[ruifortes]

👍