Support license detection for commonly used LICENSE.md files

Problem to solve

Commonly used licenses aren't properly recognized by GitLab when the license files are formatted with markdown. Evil Surround Mode's version of the GPLv3 is a good example, since GPLv3 is an incredibly common license. However, as evidenced by this project, GitLab isn't recognizing the license.

Intended users

Anyone who is more concerned with the readability of their license than its portability.

Plaintext license files are great because you can basically garuntee that they can be viewed on any system through some means - they're a normal text file, after all, which makes them super portable. However, ubiquitous formats like markdown are light enough that not having a markdown previewer isn't a huge problem, and there are an increasing amount of markdown previewers out there (including GitLab's).

Given this, many people have taken to using markdown files as license files, since they tend to be a bit better on the eyes when fully rendered.

Proposal

We could extend GitLab's API to provide markdown variants of the existing license templates, so the user could select one of these and have a pretty markdown-formatted document as their license file.

Alternatively, we could just extend the API to properly recognize markdown variants of commonly used licenses instead.