devapp/owners: render owners page HTML once
devapp/owners: render owners page HTML once

The owners table is defined in code, so it is always static until said
code is modified and rebuilt. We already execute translateOwnersPaths
just once at package initialization time. Might as well extend that to
render the entire owners page HTML once, on first HTTP request.

This makes it easier to apply transformations in code without worrying
about needing to optimize it when it's done for each HTTP request.

For golang/go#51322.

Change-Id: Ia9e49561ad28ea6f7722af83b9b0e2072299cba6
Reviewed-on: https://go-review.googlesource.com/c/build/+/387534
Trust: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Michael Pratt <e7f1edc6d5aa9f6c8c377425db8f1a5816cbbb88@google.com>

golang/build,golang/build,golang/build,golang/build