ui: Much elm work, such wow

- ui: Fix favicon
- webui: Changes to support serving Elm UI
- Static files now served from `/` mount point.
- Old UI handlers moved to `/serve` mount point, some will still be
  needed by the Elm UI; safe HTML and attachments for example.
- Update dev-start.sh for new UI, with tip on how to build it.
- ui: Detect browser host:port for websocket URL,
- webui: Remove unused mailbox handlers, rename routes
- Many routes not needed by Elm UI.
- `/serve/mailbox/*` becomes `/serve/m/*`.
- webui: Impl custom JSON message API for web UI,
- ui: Refactor Mailbox view functions,
- ui: Add body tabs for safe HTML and plain text,
- webui: Format plain text for new UI,
- ui: List attachments with view & download links,

ui/src/main.css

@@ -30,6 +30,11 @@ time, mark, audio, video {
   vertical-align: baseline;
 }
 
+a {
+    color: #337ab7;
+    text-decoration: none;
+}
+
 body {
   background-color: var(--bg-color);
 }
@@ -46,6 +51,33 @@ body, input, table {
   opacity: 1;
 }
 
+/** SHARED */
+
+a.button {
+  background-color: #337ab7;
+  background-image: linear-gradient(to bottom, #337ab7 0, #265a88 100%);
+  border: none;
+  border-radius: 4px;
+  color: #fff;
+  display: inline-block;
+  font-size: 11px;
+  font-style: normal;
+  margin: 4px;
+  padding: 3px 8px;
+  text-decoration: none;
+  text-shadow: 0 -1px 0 rgba(0,0,0,0.2);
+}
+
+.well {
+  background-color: var(--selected-color);
+  background-image: linear-gradient(to bottom, #e8e8e8 0, #f5f5f5 100%);
+  border: 1px solid var(--border-color);
+  border-radius: 4px;
+  box-shadow: 0 1px 2px rgba(0,0,0,.05);
+  padding: 4px 10px;
+  margin: 20px 0;
+}
+
 /** APP */
 
 #app {
@@ -313,6 +345,46 @@ li.navbar-active span,
   }
 }
 
+.message-body {
+  padding: 5px;
+}
+
+nav.tab-bar {
+  border-bottom: 1px solid var(--border-color);
+  display: flex;
+  margin: 20px 0 10px 0;
+}
+
+nav.tab-bar a {
+  border-radius: 4px 4px 0 0;
+  display: block;
+  margin-bottom: -1px;
+  margin-right: 2px;
+  padding: 8px 15px;
+  text-decoration: none;
+}
+
+nav.tab-bar a.active {
+  color: var(--low-color);
+  border-color: var(--border-color) var(--border-color) var(--bg-color) var(--border-color);
+  border-style: solid;
+  border-width: 1px;
+}
+
+nav.tab-bar a:focus,
+nav.tab-bar a:hover {
+  background-color: var(--selected-color);
+}
+
+nav.tab-bar a.active:focus,
+nav.tab-bar a.active:hover {
+  background-color: var(--bg-color);
+}
+
+.attachments {
+  width: 100%;
+}
+
 /** STATUS */
 
 .metric-panel {