Add MNIInfant to template enum

[tsalo]

Should help with #823.

Changes proposed in this pull request

• Add MNIInfant to allowed values for SubjectSummary's template input.
• Remove some unused constants related to templates.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 28.12%. Comparing base (8802df4) to head (4f75cf6).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #824      +/-   ##
==========================================
- Coverage   28.12%   28.12%   -0.01%     
==========================================
  Files          69       69              
  Lines       10079    10077       -2     
  Branches     1302     1107     -195     
==========================================
- Hits         2835     2834       -1     
+ Misses       7146     7145       -1     
  Partials       98       98              

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[mattcieslak]

The deletions were just unused variables, thank you!