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.

.goreleaser.yaml

@@ -22,7 +22,7 @@ builds:
       - -s -w
       - -X 'sneak.berlin/go/vaultik/internal/globals.Version={{ .Version }}'
       - -X 'sneak.berlin/go/vaultik/internal/globals.Commit={{ .Commit }}'
-      - -X 'sneak.berlin/go/vaultik/internal/globals.CommitDate={{ .CommitDate }}'
+      - -X 'sneak.berlin/go/vaultik/internal/globals.CommitDate={{ slice .CommitDate 0 10 }}'
 
 archives:
   - id: default