Changes the license from GPLv3 to AGPLv3

This changes the code's license from the GPLv3 to the AGPLv3.

### Why?
The GPL is a great copyleft license, but there is something called the ASP loophole that ends up allowing people to keep their code changes closed-source. The AGPLv3 patches said loophole, meaning that all branches and forks of an AGPLv3 codebase are legally forced to share their source code.
### What does this mean for me?
If you have a closed-source fork of /tg/station, you are not legally allowed to use any code submitted to /tg/station after the license change, unless you release the source code for your fork under the AGPLv3 license. If you are a contributor, you agree to license the code you contribute after this PR is merged under the AGPLv3, meaning that any changes to your code must also be licensed under the AGPLv3 unless you give explicit permission for it to be under another license. Otherwise, nothing changes for you.