Feature: Add shortcut option for Ignore

[erinspace]

Hi again! For one activity we had a lot of files to ignore, and this was a top request to make it easier to speed up labeling.

Description

Keyboard shortcuts are awesome and make labeling go so fast! For datasets that are a bit messier and have a lot of images to ignore, an "Ignore" keyboard shortcut would let you stay in this flow and not have to move to the mouse or use tab a bunch of times to make it to the Ignore button.

[faustomorales]

This was a thing in v0.1, but the shortcut was something like Ctrl+Shift+I or something and I felt like that was crazy. Any suggestions for a natural keyboard shortcut for "Ignore"? Implementation for this is easy -- it's the design that is a little unclear.