Search shortcut breaks forms

We have a number of forms in our mkdocs site which are broken by the search shortcut (s / f).

Expected behavior

Search only opens when not focused on a form element.

Actual behavior

When typing in a form field (e.g. input / text area) an s or f will open the search.

Steps to reproduce the bug

1. Add the following to a test page

<form>
<input></input>
<textarea></textarea>                                                                                                
</form>

2. Attempt to write sausages in the input / text area

Package versions

• Python: Python 2.7.13
• MkDocs: 0.16.3
• Material: 1.7.4

Potential fix

I think the code to fix this would be a change to: https://github.com/squidfunk/mkdocs-material/blob/master/src/assets/javascripts/application.js#L340

Something like this to check if the current focused element is part of a form:

if (document.activeElement.form === undefined) {
  if (ev.keyCode === 70 || ev.keyCode === 83) {
    query.focus()
    ev.preventDefault()
  }
}

That seems to work in testing but doesn't pass the 'flow' check and I'm not quite familiar enough with js to quickly make this work.