Allow to configure whether or not Access Request is enabled from Application Settings
Related to https://gitlab.com/gitlab-org/gitlab-ce/issues/18585
A customer mentioned this should also be a Instance-wide setting in the Administration > Settings
panel. For example, in LDAP the feature doesn't make sense sometimes:
We found that if a user requests access to a group and then is later added to the LDAP group for that project, they won't be granted access automatically since their request is still pending. We had to manually remove the LDAP sync group, decline the request, and then read the LDAP group and run the sync job again to get them added to the group via LDAP.