installation: legacy snippet configuration support

• The commit fec0eb253059b87328b2b61a04d9c6c7f41d131a changed the full-text snippet configuration from global per-word to per-doctype per-character based (for the v1.1 release series). This commit recognises the old style of configuration (the one used in v1.0 release series) and warns the user to update his/her invenio-local.conf file in case the old style is still used. Thanks to Theodoros Theodoropoulos for reporting this problem.

Co-authored-by: Tibor Simko <tibor.simko@cern.ch>