Expose project.pushRules.rejectUnsignedCommits in gql

• Expose project.pushRules in gql
• Web IDE show alert when project rejects unsigned commits

What does this MR do?

We want to alert the user in the Web IDE when the project has "reject_unsigned_commits" enabled.

This MR exposes this setting in the GraphQL API.

References

Related to #213581 (closed)