BuddyBot authorisation outcome screen

[billglover]

When users grant BuddyBot access to their Slack workspace, the authorisation flow needs to be able to communicate the result of the grant. Two pages are required that show:

• a success message when BuddyBot has been granted access to a workspace
• a failure message when BuddyBot was unable to get access to a workspace

It is unclear at this point if these should be separate static pages, or wrapped into a common page which is modified based on the outcome of the authorisation flow.