Stop naming remote-access schemes as if they were constants

A link is built by formatting that protocol's own linktemplate, and the
templates are rows in accessprotocols that a site can edit. So no scheme
is a fixed fact about the product, and writing one down as though it were
misleads whoever reads it next.

Two places did. The guest-grant docstring said the detail route resolves
"working rdp:// and vnc:// links", which is wrong for the row we actually
ship: VNC is com.realvnc.vncviewer.connect://{host}:{port}, the RealVNC
handler, not vnc://. And the protocol admin form offered vnc://{host}:{port}
as its placeholder, in the one screen where somebody authors a new template,
so the example contradicted the seeded data it sits next to.

Neither changes behaviour. _computer_access_links already reads the
template from the database and the frontend only ever renders the resolved
link, so nothing in the code assumed a scheme - only the prose did.

Scope: product

plugins/computers/frontend/views/AccessProtocolsList.vue

@@ -69,7 +69,7 @@
           </label>
           <label class="field">
             <span>Link template</span>
-            <input v-model="form.linktemplate" type="text" maxlength="255" placeholder="vnc://{host}:{port}" />
+            <input v-model="form.linktemplate" type="text" maxlength="255" placeholder="scheme://{host}:{port}" />
             <small class="muted">Placeholders: <code>{host}</code>, <code>{port}</code>, <code>{scheme}</code></small>
           </label>
           <label class="field checkbox">