Test Runs list gadget should have a limit defined by Jira administrators

Currently, the Test Runs list gadget is very flexible allowing users to query all Test Executions on a Jira system.

This wide-open behavior may bring some inconveniences to Jira like one single user blocking the Jira usage of all users.

We should limit the number of Test Runs that can be queried on the gadget and this limit should be configurable by the Jira administrators. The default value should be 2000.