gitlab: investigate how to handle multiple webhooks for the same project

[mdonadoni]

After #506, it might happen more frequently that different users have access to the same project, which will be shown to each of them in their profile page.

Currently, REANA supports only one webhook per repository, which will trigger a workflow execution only for the user that enabled the integration with GitLab. However, all the users having access to the project will see the integration as enabled in their profile page (that is, the toggle button is shown as being "on"). This can create some confusion, as users may wonder why workflows executions are not triggered even though the integration is enabled.

Some possible options:

• Keep the limit of one webhook for each project, but improve the user interface.
• Support multiple webhooks for the same project. However, we have to be careful: if many users enable the integration with the same project, there will be a lot of workflows executions requests at the same time.