mavweb: move the shell partial out of Go into shell.html (V-409)

The eight pages were already embedded .html files. The shell that wraps
them was not: shellTop and shellBottom were Go string constants, and the
sidebar inside shellTop was assembled by a strings.Builder writing
`<div class=sidebar-section>` a fragment at a time. That builder is the
markup-in-Go the review complained about.

shell.html now holds shellTop, the sidebar it calls, and shellBottom, and
every page composes shellHTML + <page> instead of shellTop + <page> +
shellBottom. Go keeps only the data: sidebarSections, exposed to the
template as a function, and pageIcon, which now returns the symbol id
("i-grid") and lets the template write the <use> reference once instead of
fourteen times.

sidebarActive was dead — nothing called it.

Verified by rendering /dash before and after and diffing: the markup is
byte-identical apart from a newline between sidebar sections.

cmd/mavweb/models.go

@@ -31,7 +31,7 @@ type modelController interface {
 	SwapModel(ctx context.Context, req ipc.SwapModelReq) (ipc.SwapModelResp, error)
 }
 
-var modelsTmpl = template.Must(template.New("models").Funcs(shellFuncs()).Parse(shellTopHTML + modelsHTML + shellBottomHTML))
+var modelsTmpl = template.Must(template.New("models").Funcs(shellFuncs()).Parse(shellHTML + modelsHTML))
 
 const modelsHTML = `{{template "shellTop" "models"}}
 <h1>Resident model</h1>