BE: Add database column for proxying self-managed instances

In #321401 (closed) we did investigation and created a prototype for a proxy of the Atlassian connect app that could be used to configure self-managed GitLab instances.

This issue is to implement the database changes described here: #321401 (closed)

Specifically:

In order to use gitlab.com as a proxy for self-managed instances, we need to know the instance URL. Therefor we need a new column instance_url on jira_connect_installations.