system: pick the management VLAN from System Settings

`vlan <id> mgmt` existed on the CLI, but nothing reported which VLAN
currently carries management, so the setting was invisible from the web UI -
the only way to find out was to read the startup config.

Report it in information.json and offer it where the other switch-wide
addressing settings live, between Gateway and Language, as a picker filled
from the configured VLANs. When management is untagged there is no VLAN to
select, so that state shows as a disabled entry rather than inventing an id
the switch would reject.

Confirm before applying, and say what will happen rather than echoing the
action back: the switch starts tagging its own frames, and if the port you
came in on does not carry that VLAN the page becomes unreachable and the way
back is the console. Cancelling puts the picker back where it was.

html/system.html

@@ -32,7 +32,7 @@
       </div>
       <div class="row">
         <div class="lcol"> <label data-i18n="sys_model">Model:</label></div>
-        <div class="rcol"><span id="model"></span></div>
+        <div class="rcol"><span id="model" class="rotext"></span></div>
       </div>
       <div class="row">
         <div class="lcol"> <label for="ip" data-i18n="sys_ip">IP address:</label></div>
@@ -46,6 +46,10 @@
         <div class="lcol"> <label for="gw" data-i18n="sys_gateway">Gateway:</label></div>
         <div class="rcol"><input id="gw" class="ip" type="text" minlength="7" maxlength="15" size="15"/></div>
       </div>
+      <div class="row">
+        <div class="lcol"> <label for="mgmtvlan" data-i18n="sys_mgmt_vlan">Management VLAN:</label></div>
+        <div class="rcol"><select id="mgmtvlan" class="ip" onchange="mgmtVlanChanged()"></select></div>
+      </div>
       <div class="row">
         <div class="lcol"> <label data-i18n="sys_language">Language:</label></div>
         <div class="rcol">