BriefingTab Fix

[DM0000]

Bad behavior on canceling "Add Mission" was due to single boolean check if mission was contract. If add mission tab was closed it would default to false and CustomizeMissionDialog frame would be generated.
-- added isMission() check

Also set briefing tab to remain visible.

Add mission check to AddMission and left BriefTab visible
closes #5060

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 10.47%. Comparing base (d333f62) to head (427e379).
Report is 21 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##             master    #5272   +/-   ##
=========================================
  Coverage     10.47%   10.47%           
- Complexity     6049     6050    +1     
=========================================
  Files           959      959           
  Lines        135202   135201    -1     
  Branches      19674    19675    +1     
=========================================
  Hits          14165    14165           
+ Misses       119686   119684    -2     
- Partials       1351     1352    +1     

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