Slack application transition statuses aren't correct

[wes-otf]

When an application is transitioned, the status displayed in the slack notification aren't reflective of the actual state and seems to just duplicate the old status twice.

Examples:

• transitioning from Needs Screening to Open Review results in a notification of <user> has updated the status of <application>: Need screening → Need screening
• transitioning from Open Review to Close Review results in a notification of <user> has updated the status of <application>: Internal Review → Internal Review

[wes-otf]

I'm still digging into this but I think it might be related to #4561 - seems the first time this popped up was around then but a staff member just noticed last week

[wes-otf]

this also is not an issue for batch transitions, it's only for individual apps