(GH-271) Move classes into extensions folder

[gep13]

Description

Consolidate all extension classes into the extensions folder.
For consistency.

Fixes #271

Related Issue

#271

Motivation and Context

To safe our sanity

How Has This Been Tested?

It hasn't, pushing to CI build

Screenshots (if appropriate):

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #272 into develop will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           develop     #272   +/-   ##
========================================
  Coverage    77.20%   77.20%           
========================================
  Files           54       54           
  Lines         1132     1132           
  Branches       135      135           
========================================
  Hits           874      874           
  Misses         237      237           
  Partials        21       21           

Impacted Files	Coverage Δ
...easeManager.Core/Extensions/MilestoneExtensions.cs	0.00% <ø> (ø)
...eleaseManager.Core/Extensions/OctokitExtensions.cs	0.00% <ø> (ø)
...leaseManager.Core/MappingProfiles/GitHubProfile.cs	0.00% <ø> (ø)

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