feat: kadabra, the app auto-updater (!268)

coop-cloud/organising#236

Autoupdater `kadabra` is ready for testing.
It should run on the server, check for available minor/patch updates and automatically upgrade the apps.

Co-authored-by: Moritz <moritz.m@local-it.org>
Reviewed-on: coop-cloud/abra#268

Makefile

@@ -1,4 +1,5 @@
 ABRA    := ./cmd/abra
+KADABRA := ./cmd/kadabra
 COMMIT  := $(shell git rev-list -1 HEAD)
 GOPATH  := $(shell go env GOPATH)
 LDFLAGS := "-X 'main.Commit=$(COMMIT)'"
@@ -18,9 +19,11 @@ build-dev:
 
 build:
 	@go build -ldflags=$(DIST_LDFLAGS) $(ABRA)
+	@go build -ldflags=$(DIST_LDFLAGS) $(KADABRA)
 
 clean:
 	@rm '$(GOPATH)/bin/abra'
+	@rm '$(GOPATH)/bin/kadabra'
 
 format:
 	@gofmt -s -w .