fix: Disable mail system

[orronai]

• Added TESTING attribute to the configuration file which disables the mail system.
• Added instructions to the README file.

[codecov]

Codecov Report

Merging #316 (ba5adb6) into master (33c8dc8) will increase coverage by 0.02%.
The diff coverage is 93.33%.

❗ Current head ba5adb6 differs from pull request most recent head 42e1fa3. Consider uploading reports for the commit 42e1fa3 to get more accurate results

@@            Coverage Diff             @@
##           master     #316      +/-   ##
==========================================
+ Coverage   84.17%   84.20%   +0.02%     
==========================================
  Files          60       60              
  Lines        2541     2552      +11     
==========================================
+ Hits         2139     2149      +10     
- Misses        402      403       +1     

Impacted Files	Coverage Δ
lms/utils/mail.py	97.22% <93.33%> (-2.78%)	⬇️

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[yammesicka]

Maybe MAIL_DISABLED? Or just check that MAIL_SERVER is None?

[orronai]

https://pythonhosted.org/Flask-Mail/#unit-tests-and-suppressing-emails
The another option is MAIL_SUPRESS_SEND

[yammesicka]

We shouldn't change a "TESTING" variable in our configuration file manually.
If we want, we should adjust it in the conftest.py or add a pytest script that would do it for us.
We can add a DISABLE_MAIL that uses this variable -- but this name for a configuration variable is problematic