Prevent setting alert to null for valid gesture

[shresthabijay]

fixes #665

[shresthabijay]

@raineorshine The issue was specific to mobile because alert was set to null at handleGestureEnd without checking if the gesture was valid.

em/src/shortcuts.js

Line 135 in 06ad37b

setTimeout(() => alert(null))

So whenever delete gesture was complete, archiveThought showed undo alert and immediately after that handleGestureEnd set it to null.

I made changes so that alert is set to null inside handleGestureEnd only if the gesture is not valid. Please review the changes. Thank you!

[raineorshine]

That's definitely the cause, but unfortunately we need to clear the gesture hint when the gesture is valid.

In the process of investigating, I stumbled upon a solution. Will you review? #677