Move to AGPLv3-only
We were really fast on !35 (merged) that we did not notice that the AGPL would better suit the needs of elixi.re.
This MR also changes the headers on source code files, from the "GPL headers" to SPDX, which are small and machine-readable (The basis of it is a comment with
SPDX-License-Identifier, which is simple to check against, compared to a long comment block explaining the meat of a license).
- Remove GPLv3 headers
Add SPDX headers with
LICENSE.spdxwith legal information (such as creators, etc)?
- Contact contributors for confirmation of license change