Cannot set repository to pull mirror
### Summary
In self-hosted GitLab Enterprise Edition 13.5.3-ee, unable to create pull mirror: pull option is greyed out.
### Steps to reproduce
Have a self hosted instance with EE license. Create a new repository that is imported by URL from another repository.
### Example Project
Self hosted: N/A
### What is the current *bug* behavior?
Navigate to Settings -> Repository, expand Mirroring repositories.
Enter URL of remote repository (http://osm.etsi.org/osm/devops.git).
Attempt to change Mirror direction, but cannot. Push is shown in grey and cannot be changed.
### What is the expected *correct* behavior?
Able to select mirror direction of Pull.
### Relevant logs and/or screenshots

### Output of checks
This bug happens on self-hosted GitLab
#### Results of GitLab environment info
#### Results of GitLab application Check
### Possible fixes
issue