feat(panel): navigation as a left column, with a section index on the long pages

The navigation was a bar across the top that did not fit on one row — six page
entries and the session block against the panel's width — and had to be split
into two, costing the top of every page. It is now a column down the left edge:
one left edge to scan, the current entry marked down its leading edge, sticky so
it stays in view, and room under the entries for the current page's own
sections. Below the width the two columns need it lies back down into the same
wrapping rows as before; six entries need no drawer.

The section index is for the two pages long enough to need one — the domain page
(nine cards) and the status page (eight). Each card carries an id and the page's
template defines the list by overriding an empty "sections" block in the layout,
so a page that defines nothing renders no index. panel.js marks the section in
view, looking targets up by id on each pass so the status page swapping its
cards out every five seconds cannot leave it measuring boxes that have left the
document; the links themselves are plain fragment links and need no script.

Verified against the real pages rendered by a local panel at 1300px, 924px and
481px wide.

internal/web/templates/status.html

@@ -5,7 +5,7 @@
 
 {{template "status_body" .}}
 
-<div class="card">
+<div class="card" id="hostname">
   <h2>Hostname and reverse DNS <span class="st st-{{.PTR.Status}}">{{.PTR.Status}}</span></h2>
   <p class="muted">Receiving servers check that the name this server announces
   resolves to its address <em>and</em> that the address resolves back to the same
@@ -29,7 +29,7 @@
   </form>
 </div>
 
-<div class="card">
+<div class="card" id="configuration">
   <h2>Configuration</h2>
   <p class="muted">Regenerates the OpenDKIM and Postfix configuration from the
   database and reloads both daemons. Use it if you edited the files by hand,
@@ -41,3 +41,22 @@
   </form>
 </div>
 {{end}}
+
+{{/* The status page's section index, shown in the navigation column (see the
+     "sections" block in layout.html). The first six cards are the ones the
+     polling fragment replaces every five seconds; their ids are part of
+     status_body.html and do not change with the reading, so the links here hold
+     across a refresh. */}}
+{{define "sections"}}
+<div class="sections">
+  <p class="sections-title">On this page</p>
+  <a href="#overall">Overall</a>
+  <a href="#processes">Processes</a>
+  <a href="#machine">Machine</a>
+  <a href="#queue">Mail queue</a>
+  <a href="#certificate">TLS certificate</a>
+  <a href="#sockets">Milter sockets</a>
+  <a href="#hostname">Hostname and reverse DNS</a>
+  <a href="#configuration">Configuration</a>
+</div>
+{{end}}