Remove mousetrap-rails

Remove mousetrap-rails gem because:

• the gem has never been maintained for about 5 years (which prevents us to update the upstream JS while the upstream JS package is still updating)
• we do not need the JS wrapper the gem provides
• including the gem takes 268 kB, which we can save by removing the gem

Proposal

Remove mousetrap-rails gem safely as we are already relying directrly on mousetrap.