Add License Management API to the backend

What does this MR do?

It adds approval and blacklist API for software licenses; see https://gitlab.com/gitlab-org/gitlab-ee/issues/5488

Why was this MR needed?

To implement https://gitlab.com/gitlab-org/gitlab-ee/issues/5488

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?

Refs #5488 (closed)

added 1 commit

• 2c2b1a51 - Adds tests.

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• 616f0d93 - Cleanup, more work on tests.

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added 1 commit

• 9b412872 - Fixes all controller tests.

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added 1 commit

• 91764835 - Fixes all API tests.

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All RSpec tests are ok. I'll work on the API documentation, and on moving EE code to appropriate places.

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• 347fed58 - Move files to ee

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added 4 commits

• f705ea46 - Cleanup
• 29a601ef - Cleanup
• a668a992 - Adds :id field of managed_license objects in API responses
• 698c9fa5 - Adds API documentation

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added 1 commit

• 85eef045 - Remove unneeded index

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resolved all discussions

marked the checklist item Changelog entry added, if necessary as completed

marked the checklist item Documentation created/updated as completed

marked the checklist item API support added as completed

marked the checklist item EE specific content should be in the top level /ee folder as completed

marked the checklist item Conform by the merge request performance guides as completed

marked the checklist item If you have multiple commits, please combine them into a few logically organized commits by squashing them as completed

added 11 commits

• b9adfbb1 - Adds Changelog. Cleanup specs.
• d8378d5a - Add some tests for the API
• e1987ba7 - Add managed license doc to README, add Ultimate
• 03b75543 - Change "repository" to "project"
• cc767a3d - Cleanup
• 374db50c - Fixes style
• 1d07e970 - WIP
• e169b1d1 - Cleanup, comments.
• 3e15d5e2 - DRY
• 774d9bd2 - Add comments
• 03e9330e - Update comments

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marked the checklist item Conform by the style guides as completed

marked the checklist item Tests added for this feature/bug as completed

added 1 commit

• 5646faf5 - Fixes file name

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