Add doesUsenameExist query

src/operations/user.gql

@@ -131,3 +131,7 @@ query getUserFollowingFollowers {
     }
   }
 }
+
+query doesUsernameExist($candidateUsername: String!) {
+  doesUsernameExist(candidateUsername: $candidateUsername)
+}

src/schema.gql

@@ -15,20 +15,17 @@ type Query {
     ids: [Int!] = null
     shotsOrdering: GetShotsOrdering = null
     limit: Int! = 500
-    countRespectsLimit: Boolean! = false
   ): GetShotsResult!
   getShotsWithMetadata(
     filterInput: FilterInput!
     ids: [Int!] = null
     shotsPagination: GetShotsPagination = null
     limit: Int! = 500
-    countRespectsLimit: Boolean! = false
   ): GetShotsResult!
   getShots(
     filterInput: FilterInput!
     shotsPagination: GetShotsPagination = null
     limit: Int! = 500
-    countRespectsLimit: Boolean! = false
   ): [ShotGQL!]!
   getShotsByIds(ids: [Int!]!): [ShotGQL!]!
   getShotAnnotationTypes(errorTypes: Boolean = false): [ShotAnnotationTypeGQL!]!