Add slash command for updating incident severity

Release Notes

Part of our incident workflow for GitLab.com is to update severity levels frequently. We do this right now with ~severity:: labels and slash commands since it always accompanies a note, for example:

lowering severity because of reason XX

/label ~"severity::3" 

When we transition over to GitLab incidents we will likely want to do the same with incident severity. I don't think we yet have special slash commands for incidents, it might be something like

/severity 3