modules/web_hook: Add post-push handler for reacting to push events.

modules/client/keys/claim: Handle local users without loopbacking.

modules/federation/user_keys_query: Cross signatures.

ircd::cl: MESA_GLSL_CACHE_DISABLE is deprecated.

ircd::cl: Stop propagation of platform and device initialization errors.

docker: Refactor into build matrix. [ci skip]

Update README; link to wiki for docker grid. [ci skip]

appveyor: Skip extra builds for tags; minor cleanup.

ircd::net::acceptor: Remove backreference to wrapper object.

ircd::m::vm::execute: Fix propagation of conformity exceptions masked…

… by opts.