Fix Internal Server error from branchRuleCreate mutation

What does this MR do and why?

Contributes to #508212 (closed)

Problem

The presence check for project is missing for branchRuleCreate mutation. Also, the permission to create a branch rule is not enforced on mutation level.

Solution

Use authorized_find! to verify the presence of provided project path and user permissions to create a branch rule.

References

GraphQL 500 Internal Server Error : branchRuleC... (#508212 - closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

Steps to Reproduce

1. Execute the GraphQL mutation branchRuleCreate as shown below:

   mutation branchRuleCreate($input: BranchRuleCreateInput!) {
     branchRuleCreate(input: $input) {
       branchRule {
         branchProtection {
           allowForcePush
         }
       }
       clientMutationId
       errors
     }
   }

2. Use the following variables:

   {
     "input": {
       "projectPath": "859",
       "name": "random_string_281"
     }
   }

3. Observe that the system doesn't return a 500 status code.

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Related to #508212 (closed)