We now detect whether a file is being dropped by a user (by checking `event.dataTransfer.files.length`) and prevent the remaining drop behavior if this is the case. Otherwise, drop happens like normal (so a user can still drop text into the editor, or even an image that has been uploaded already via normal means and rendered in the editor).

This fixes an issue where mixing strong and em text caused the text
to not render correctly as a post (e.g. *This is **bold text**.*
would fail).

Previously, when editing a post with an unordered list in it via the
Classic editor, backslashes (\) would get added to the end of each list
item. This fixes that.

Closes #480

Ref T727

Ref T727

This uses String.replace() instead of String.replaceAll().

This upgrades @babel/preset-env to 7.9.0 from 7.8.7.

See: https://github.com/nodejs/node/issues/32852#issuecomment-613655150