LP#1098669: improve handling of Content Cafe external content in staff client

LP#1098669: improve handling of Content Cafe external content in staff client

This patch introduces a new TPac content variable, ctx.ext_proto, to
specify whether external content should be retrieved via HTTP or HTTPS.
ctx.proto can't be used for that purpose since it's set to 'oils' when
TPac is running inside the staff client.

This fixes a bug where Content Cafe reviews wouldn't display in the staff
client bib record page and where clicking on the reviews link from search
results would open an empty window.

Signed-off-by: Galen Charlton <gmc@esilibrary.com>