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>