Limit headways at end of service

[PaulJKim]

Summary of changes

Asana Ticket: Limit Headway display during end of service

Change to stop showing headway messages slightly before the last scheduled trip to be a little more accurate.

Also added a 1 min buffer to both the start of headway messages as well as the end to be slightly more accurate. For example, if the last scheduled train is at 1:00AM and the low range of headways is 5 minutes, then it would still be fair to show headways until the clock changes to 12:56AM, but 12:56AM would be [time of the last scheduled train] - 4 minutes, rather than 5 minutes.

[github-actions]

Coverage of commit 26acb85

Summary coverage rate:
  lines......: 74.5% (2021 of 2713 lines)
  functions..: 79.8% (557 of 698 functions)
  branches...: no data found

Files changed coverage rate:
                                                             |Lines       |Functions  |Branches    
  Filename                                                   |Rate     Num|Rate    Num|Rate     Num
  =================================================================================================
  lib/engine/scheduled_headways.ex                           |97.7%     87|88.9%    18|    -      0
  lib/signs/utilities/headways.ex                            |95.2%     21| 100%     3|    -      0

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[panentheos]

Looks good!