Add index on search terms for event log table

It is possible to add an expression index in Postgres: https://www.postgresql.org/docs/current/indexes-expressional.html

Since we support a simple text search on multiple columns by combining their content we could add a dedicated index for the expression we are using.
Unfortunately this requires some additional work because the `text` function we use for casting JSON metadata is locale-dependent which makes the expression not immutable.