Fix: Installs whose number has been re-used as an NN shouldn't be able to get an NN of their own

[Andrew-Dickinson]

Fixes an issue where after an old install number is recycled as a network number for another node, that old install number is still able to get an NN of its own. This is needlessly confusing and creates extra edge cases when trying to disambiguate a number that is presented into a NN or Install Number. It's not a big deal to ask these folks to just fill out the join form again, so we disallow this from happening

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.88%. Comparing base (c285614) to head (75aeb1d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #802   +/-   ##
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  Coverage   94.88%   94.88%           
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  Files          89       89           
  Lines        3809     3811    +2     
=======================================
+ Hits         3614     3616    +2     
  Misses        195      195           

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