internal/static: minify generated files in watch mode
internal/static: minify generated files in watch mode

To prevent confusion around uncommitted changes when running
pkgsite in dev mode we can minify the generated static files.
Given the speed of the esbuild tool we should see little to
no impact in the build times while making JS or CSS
changes locally.

Change-Id: I2e5f7feaa7da5ae069ce9acdd16578d1c94ef8ea
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/365656
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite