chore: relicense to GPL-3.0-or-later

[MotherSphere]

Colony shipped a verbatim MIT licence text while every other statement of
the project's terms was either absent or a copy of the same MIT claim, so
there was no machine-readable licence at all: cargo metadata, crates.io
tooling and downstream packagers saw a crate with no license field, and
the only signal was prose. Moving the project to the GNU GPL v3 or later
means all of those statements have to agree, and the missing one has to
exist.

• LICENSE: replaced the MIT text with the verbatim FSF GPL-3.0 text. The
  file is identical for -only and -or-later; the version choice is
  carried by the SPDX strings and the prose below, not by this file.
• Cargo.toml: added license = "GPL-3.0-or-later". The package had no
  license (nor license-file) field whatsoever, so the crate metadata
  contradicted the LICENSE file by saying nothing.
• README.md: the shields.io badge hardcodes the licence name inside the
  image URL, so it rendered "License: MIT" no matter what LICENSE said;
  repointed it at GPL-3.0-or-later. The License section now names the new
  terms and spells out the "or (at your option) any later version" grant,
  which is the only place a reader can tell -or-later from -only.
• CONTRIBUTING.md: the inbound-licence clause told contributors their work
  was accepted under MIT. Left unchanged it would have kept taking in
  contributions under terms the project no longer distributes under.
• docs/faq.md: "Is Colony free / open source?" answered "MIT licensed".

Third-party licences are untouched: src/ui/assets/fonts/JetBrainsMonoNerdFont/OFL.txt
(SIL OFL 1.1, JetBrains Mono) covers a bundled font, not Colony's own code.
The MIT string in src/ui/markdown_blocks.rs is a test fixture for the badge
parser that renders other repositories' READMEs, not a claim about Colony.