Improve Teaching Merging + Email Management

[cycomachead]

• Cleans up the initial teachers request from too
• Fixes up merging records to handle primary email addresses better
• displays a 'Primary' label on the teacher's #show page and table views
• uses eager loading for table views (including merging) so the database isn't hammered.

[codecov]

Codecov Report

Attention: Patch coverage is 86.20690% with 4 lines in your changes missing coverage. Please review.

Project coverage is 89.24%. Comparing base (c1d35ea) to head (43b9c39).

Files	Patch %	Lines
app/controllers/merge_controller.rb	85.71%	2 Missing ⚠️
app/controllers/sessions_controller.rb	80.00%	1 Missing ⚠️
app/models/teacher.rb	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
- Coverage   89.52%   89.24%   -0.29%     
==========================================
  Files          28       28              
  Lines         936      958      +22     
==========================================
+ Hits          838      855      +17     
- Misses         98      103       +5     

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