Add author/homepage/license to version + banner; date format fixes

- globals.go: add Homepage and License constants.
- version command: show author, homepage, license, build date.
- Startup banner reformatted to:
    vaultik X by Author (commit Y, built on Z) starting up at T.
    https://sneak.berlin/go/vaultik
- Commit date now formatted as YYYY-MM-DD (called "build date" in
  user-facing output, since the binary was at least compiled once on
  the date of commit). Makefile/Dockerfile use git --format=%cs.
  goreleaser slices its RFC3339 .CommitDate template var to 10 chars.

Makefile

@@ -5,7 +5,7 @@ VERSION := 1.0.0-rc.1
 
 # Build variables
 GIT_REVISION := $(shell git rev-parse HEAD 2>/dev/null || echo "unknown")
-GIT_COMMIT_DATE := $(shell git show -s --format=%cI HEAD 2>/dev/null || echo "unknown")
+GIT_COMMIT_DATE := $(shell git show -s --format=%cs HEAD 2>/dev/null || echo "unknown")
 
 # Linker flags
 LDFLAGS := -X 'sneak.berlin/go/vaultik/internal/globals.Version=$(VERSION)' \