This commit adds copyright attributions in SPDX to the files for matrix-cactus-comments and matrix-cactus-comments-client, following REUSE's specification.

Signed-off-by: Suguru Hirahara <acioustick@noreply.codeberg.org>

This is split out from matrix-cactus-comments (see 241779b583),
but also heavily inspired by `matrix-static-files`.