Added custom tls cert support

[Spitfireap]

• Allow user to set a pre-generated SSL cert (manual certificate added)
• Added information regarding LetsEncrypt ToS

[codecov]

Codecov Report

Attention: Patch coverage is 17.85714% with 23 lines in your changes are missing coverage. Please review.

Project coverage is 53.10%. Comparing base (7dcf69b) to head (e9a41dd).
Report is 31 commits behind head on master.

❗ Current head e9a41dd differs from pull request most recent head 18369e2. Consider uploading reports for the commit 18369e2 to get more accurate results

Files	Patch %	Lines
modoboa_installer/ssl.py	12.50%	21 Missing ⚠️
modoboa_installer/utils.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #504      +/-   ##
==========================================
- Coverage   54.40%   53.10%   -1.30%     
==========================================
  Files          10       10              
  Lines         761      789      +28     
==========================================
+ Hits          414      419       +5     
- Misses        347      370      +23     

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[Spitfireap]

Thanks to "jiwisd" on discord for the test.