release: 0.4.0: Domain-admin groundwork, send-log auth, packaging fixes

The project's mark is now in use rather than only on file. The README opens
Added:
  - The project's mark is now in use rather than only on file. The README opens
Changed:
  - panel: the sign-in and setup pages are now a column the width of their own
  - panel: the three monitoring pages — Deliveries, Mail queue, System log — are
Fixed:
  - panel: Deliveries now shows the subject as text rather than as its MIME
  - panel: table cells may now break inside a word, so no single long value can
  - panel: the Applications list on a domain page no longer comes apart. It was a

Full detail: CHANGELOG.md (0.4.0).

internal/web/templates/login.html

@@ -1,4 +1,8 @@
 {{define "content"}}
+{{/* The full mark, on the two pages that carry no navigation: with no bar above
+     it, this is the only thing telling an administrator which service just asked
+     them for a password. */}}
+<img class="mark" src="/static/logo.svg" width="330" height="150" alt="SelfPost">
 <h1>Sign in</h1>
 <div class="card narrow">
   {{if .SetupHint}}