fix: accept std "To" addrs in announcement tool

[jennifer-richards]

The "To" dropdown on the secr app announcement tool has known-valid addresses, so do not validate them. The to_custom field is adequately validated by the MultiEmailField for now. Limit custom validation to the cc field.

The entire form needs to be reworked, so this is just a quick fix for the immediate problem(the "To" options in the dropdown are rejected with a confusing error message)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.66%. Comparing base (6fcc7d8) to head (0a9a7db).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #11524      +/-   ##
==========================================
+ Coverage   88.59%   88.66%   +0.06%     
==========================================
  Files         333      333              
  Lines       44728    44963     +235     
==========================================
+ Hits        39626    39865     +239     
+ Misses       5102     5098       -4     

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