The IsAdmin flag is set based on whether the admin filter
returned any result. The admin filter is applied with the user dn
as the search root.

In the future, we should update IsAdmin as well on each login.
Alternately, we can have a periodic sync operation.

The same logic was duplicated in three places. Factor it
out so that we can add further fallbacks in a single place.

423b1c69be7828c331a47467b66217d190e916b9, which in turn
causes all content to be served as text/plain. Let go decide
what the mime type is (e.g. text/html instead of text/plain).

423b1c69be7828c331a47467b66217d190e916b9, which in turn
causes all content to be served as text/plain. Let go decide
what the mime type is (e.g. text/html instead of text/plain).