Feature/187044317 add email resend flash message

[JacksonXu33]

[Pivotal Tracker Link][tracker]

https://www.pivotaltracker.com/n/projects/2406982/stories/187044317

What this PR does:

This pull request implements adding a flash message popup indicating whether the welcome email resend was a success or failure. A side note is that the new resend_welcome_email in the teachers_controller.rb now causes a new HTTP request to get sent after clicking resend, whereas the old one did not specify a redirect or render.

Include screenshots, videos, etc.

How should this PR be tested?

I added some RSpec tests to test for the flash message and redirect depending on verification status of account. Also manually tested with different types of accounts on the website to check flash message after clicking resend welcome email.

• Is there a deploy we can view?
  No, but it's relatively simple code.

• What do the specs/features test?
  Checking flash hash based on condition of teacher, and checking redirect afterwards.

• Are there edge cases to watch out for?
  No.

Are there any complications to deploying this?

No

Checklist:

• Has this been deployed to a staging environment or reviewed by a customer?
• Tag someone for code review (either a coach / team member)
• I have renamed the branch to match PivotTracker's suggested one (necessary for BlueJay) (e.g. michael/12345-add-new-feature Any branch name will do as long as the story ID is there. You can use git checkout -b [new-branch-name])

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.03%. Comparing base (2ee19ed) to head (3aefd1e).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #31      +/-   ##
==========================================
+ Coverage   81.95%   82.03%   +0.08%     
==========================================
  Files          20       20              
  Lines         654      657       +3     
==========================================
+ Hits          536      539       +3     
  Misses        118      118              

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[ArushC]

Looks good to me. The RSpec tests seem to confirm that the feature works as expected. If anyone else wants to take a look, feel free, but otherwise I think this is ready to be merged into main.