Use official markdown license for AGPLv3.

[iphydf]

Taken from https://www.gnu.org/licenses/agpl-3.0.md

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This change is 

[robinlinden]

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Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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[piling]

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Reviewed 2 of 2 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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[fcore117]

Reviewed 2 of 2 files at r1, 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable

[robinlinden]

Reviewed 1 of 1 files at r2.
Review status: all files reviewed at latest revision, all discussions resolved, some commit checks failed.

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Comments from Reviewable