internal/frontend: limit synopsis on search page
internal/frontend: limit synopsis on search page

The package synopsis for each package on the search page is limited to
two lines. An ellipsis is displayed on the second line if the synopsis
is longer.

Fixes golang/go#44174

Change-Id: I8484a3defeea23285e47e6e15de3d2d4595aca26
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/290549
Trust: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
Run-TryBot: Julie Qiu <8d32267b6b4884cf35adeaccde2b6857ae11aace@golang.org>
TryBot-Result: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Jamal Carvalho <e0195770807aa8c82b0b128d9c0423b5ad035172@golang.org>

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