Describe the pull request

Adds support to command line creation of repository. This is useful for user and repos creation in bulk.
Link to the issue: #5697

Checklist

• [ x] I agree to follow the Code of Conduct by submitting this pull request.
• [ x] I have read and acknowledge the Contributing guide.
• [ x] I have added test cases to cover the new code or have provided the test plan.

Test plan

• Make sure a user is available in database. If not, use:

./gogs admin create-user --name=user3 --password=mypassword --email=user3@user.com --admin=false

• Run the following sentence in command line:
  ./gogs admin create-repository --username=user3 --repository_name=repo

Expected to get the repository created.

• Re-run the repository creation and should get a duplication error.
• Run the repository creation for an invalid user and must get an error No user was found with ...