release: 1.5.0: Send-log retention configurable in panel Settings

Send-log retention is configurable from panel **Settings** (global administrator); `SEND_LOG_RETENTION_DAYS` seeds the initial default only.
Added:
  - Panel **Settings** (global administrator): **Send log retention (days)** —
Changed:
  - Deliveries list and delivery detail pages show the configured retention

Full detail: CHANGELOG.md (1.5.0).

internal/web/view/templates/deliveries.html

@@ -8,6 +8,10 @@
 {{define "content"}}
 <h1>Deliveries</h1>
 
+<p class="muted">Rows older than {{.SendLogRetentionDays}} days are deleted
+automatically.{{if .IsGlobal}} Change retention on
+<a href="/settings">Settings</a>.{{end}}</p>
+
 <div class="card">
   <h2>Filter</h2>
   <form method="get" action="/deliveries">