Update Extempore backend to use new banner template/functionality

[oubiwann]

Tasks:

• [x] Add logic for checking during the start phases
  • [x] Only display the banner there if the backend is expected to be ready at this point in the startup sequence
  • [x] Add configuration for banner display; maybe banner-time? and at-startup or at-connect?
• [ ] Add logic for at-connect in xt.msg
• [ ] Add template variable interpolation (like is now done for the Bevin backend)
• [x] Remove any old code for the old approach to banner display

Part of #92

[oubiwann]

Done.