Refactor pr reminder v2

[khalford]

Description:

Rewrite of PR reminder feature:
Merged pr_reminder.py, post_to_dms.py and base_feature.py into one module. The base feature class is not needed anymore as future features will not be inheriting any methods from it, therefore it makes sense to remove it.

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Submitter:

Have you done the following?:

• Labeled the pull request from the following? major | minor | patch or documentation | workflow
• Updated the documentation and template config files?
• Added unit tests for new / untested code?
• Verified the relevant staging Docker image runs on dev-cloud-chatops.nubes.rl.ac.uk?

New / Existing features:

• Written integration tests?
• Configured the Slack Application with new commands or permission scope changes?
• Updated the version.txt and docker-compose.yml files?

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Reviewer:

Have you checked the following?:

• Version change is appropriate to the code changes? Semantic versioning documentation here
• Unit test code coverage is acceptable?
• Do the CI jobs pass?

[codecov]

Codecov Report

Attention: Patch coverage is 98.98649% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.52%. Comparing base (1ebec0e) to head (7be42ac).
Report is 24 commits behind head on master.

Files with missing lines	Patch %	Lines
cloud-chatops/tests/test_pr_reminder.py	97.72%	2 Missing ⚠️
cloud-chatops/tests/test_find_prs.py	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   94.97%   96.52%   +1.55%     
==========================================
  Files          19       13       -6     
  Lines         776      634     -142     
==========================================
- Hits          737      612     -125     
+ Misses         39       22      -17     

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[anish-mudaraddi]

LGTM - had a chat with @khalford for how to refactor this code for extensibility so that would be the next step as we extend this to look at gitlab or send messages to places other than slack