fix(dnd): ignore non-file drag'n'drop events

[ramusus]

Since we only deal with files, it makes sense to ignore all events non-file related (eg. dragging
plaintext). This commit fixes a few things that have changed in the browsers which subtly break
the current checks.

• The contains function on dt.files has been removed from the spec and will always return
  undefined. Except for IE, which hasn't implemented the change.

  • Chrome and Firefox have replaced it with includes, which we now use
  • We've left a contains check in there for IE as a last resort
  • Remove the comment about it being Firefox only, since it also works in Chrome now
  • More info re: removal at: https://github.com/tc39/Array.prototype.includes#status

• The dt.files property always seems to be an empty array for non-drop events. Empty arrays are
  truthy, and so this will always satisfy the isValidFileDrag check before it can validate that
  the types array includes files

  • It will now only be truthy if the files array actually contains entries

• There is a drop handler which binds to the document and always prevents all default drop
  behaviour from occurring, including things like dropping text into textfields

  • It will now only prevent default behaviour for file drops, which has the handy side-effect
    of preventing the page from navigating to the dropped file if the user misses the dropzone.

Fixes FineUploader#1588.

Brief description of the changes

{also describe what problem(s) these changes solve & reference any related issues/PRs}

What browsers and operating systems have you tested these changes on?

{example: Safari on iOS 9.1.0 and IE11 on Windows 8.1}

Have you written unit tests? If not, explain why.

{unit tests should accompany almost all PRs, unless the change is to documentation}