static/shared/form: standardize select element for all browsers
static/shared/form: standardize select element for all browsers

Created a set of styles for select elements that overrides
the browser default so it looks the same for all users and
fits into an input group.

Change-Id: Id271661d6b2da1acef0e13f08903d45d715829f1
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/340929
Trust: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
Run-TryBot: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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