Add support for keyboard shortcuts

[alanmoo]

This may need to be broken into many smaller issues, but for now I want to collect and discuss them here

Tasks that come to mind:

• [ ] Changing channels
• [x] Navigating through message command lists (typing / in the message bar)

There are clearly opportunities for many more. Comment below if you have a specific one in mind

[alanmoo]

Looks like navigating message commands is already in place in a later build of Mattermost than we have running.

[cadecairos]

Upstream bug: https://mattermost.atlassian.net/browse/PLT-215