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.

internal/globals/globals.go

@@ -19,6 +19,12 @@ var CommitDate string = "unknown"
 // Author identifies the upstream author of vaultik.
 const Author = "Jeffrey Paul <sneak@sneak.berlin>"
 
+// Homepage is the canonical URL for vaultik.
+const Homepage = "https://sneak.berlin/go/vaultik"
+
+// License is the SPDX identifier for the project license.
+const License = "MIT"
+
 // Globals contains application-wide configuration and metadata.
 type Globals struct {
 	Appname    string