legacy: fix websearch unformatted vars stacktrace

* Change 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>
Merge branch 'fix-websearch-string-formatting' of https://github.com/dset0x/invenio into fix-websearch-string-formatting