feat: Add session mails and notify endpoint

[iamareebjamal]

Fixes fossasia/open-event-frontend#4604

[codecov]

Codecov Report

Merging #7198 into development will decrease coverage by 0.04%.
The diff coverage is 51.16%.

@@               Coverage Diff               @@
##           development    #7198      +/-   ##
===============================================
- Coverage        63.02%   62.98%   -0.05%     
===============================================
  Files              259      259              
  Lines            13013    13042      +29     
===============================================
+ Hits              8201     8214      +13     
- Misses            4812     4828      +16     

Impacted Files	Coverage Δ
app/api/routes.py	100.00% <ø> (ø)
app/api/helpers/mail.py	25.40% <33.33%> (-0.24%)	⬇️
app/api/sessions.py	57.55% <47.05%> (-1.20%)	⬇️
app/models/helpers/versioning.py	89.28% <50.00%> (-6.72%)	⬇️
app/api/helpers/permission_manager.py	58.53% <57.14%> (+0.14%)	⬆️
app/api/schema/sessions.py	90.24% <66.66%> (-3.00%)	⬇️

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[lgtm-com]

This pull request fixes 1 alert when merging 89c9da2 into f609453 - view on LGTM.com

fixed alerts:

• 1 for Unused import