Create Feature Flag for reject_unsigned_commits_by_gitlab

Description

In #213581 (closed) we added a feature for the Web IDE that prevents used from committing when the project has "Reject unsigned commits" checked. Since the Web IDE cannot sign commits, this is consistent with the rule's intention.

Unfortunately, until #19185 (closed) is resolved, this has created a temporary inconvenience to self-hosted instances that wish to trust commits created through GitLab #213581 (comment 537289348).

Proposal

• Let's create a feature flag :reject_unsigned_commits_by_gitlab, which is defaulted to ON.
• If this feature flag is OFF, we will ignore the reject_unsigned_commits check in the Web IDE.

ORIGINAL:

• Let's create a feature flag :allow_unsigned_commits_by_gitlab, which is defaulted to OFF.
• If this feature flag is ON, we will ignore the reject_unsigned_commits check in the Web IDE.