Add an endpoint to display slow blazegraph queries

**Motivation**
Slow blazegraph queries submitted by users are currently saved in database for later investigation.

However, there is currently no other way to get them but do a query in the database.
The idea is to introduce an endpoint so that in the end, we can have a page in Fusion to display them.

**Acceptance criteria**
1. The most recent queries are displayed first
2. No pagination is required for now as we don't have a lot of them.
3. The default threshold to mark a query as slow can be lowered