Add antispam plugin support + OCR example

CHANGELOG.md

Dockerfile

cmd/media_repo/reloads.go

cmd/plugin_antispam_ocr/main.go

common/config/conf_main.go

common/config/models_main.go

common/config/watch.go

common/globals/reload.go

common/runtime/init.go

config.sample.yaml

controllers/upload_controller/upload_controller.go

go.mod

go.sum

matrix/admin.go

matrix/auth.go

matrix/client_server.go

plugins/manager.go

plugins/mmr_plugin.go

plugins/plugin_common/handshake.go

plugins/plugin_interfaces/antispam.go