legacy: fix websearch unformatted vars stacktrace

• Changes output generation of perform_index to use .format(). Requesting the admin interface page for WebSearch previously always resulted in a stacktrace due to unformatted strings.

Signed-off-by: Dimitrios Semitsoglou-Tsiapos <dsemitso@cern.ch>
Reviewed-by: Jan Aage Lavik <jan.age.lavik@cern.ch>