Move a very specific extension to the one file that uses it

contributes to iOS-319

MastodonIntent/Handler/SendPostIntentHandler.swift

@@ -57,7 +57,7 @@ extension SendPostIntentHandler: SendPostIntentHandling {
                 }
                 mastodonAuthentications = [authentication]
             } else {
-                mastodonAuthentications = try accounts.mastodonAuthentication()
+                mastodonAuthentications = try accounts.mastodonAuthentications()
             }
 
             let authenticationBoxes = mastodonAuthentications.map { authentication in
@@ -152,3 +152,15 @@ extension SendPostIntentHandler: SendPostIntentHandling {
     }
 
 }
+
+extension Array where Element == Account {
+    @MainActor
+    func mastodonAuthentications() throws -> [MastodonAuthentication] {
+        let identifiers = self
+            .compactMap { $0.identifier }
+            .compactMap { UUID(uuidString: $0) }
+        let results = AuthenticationServiceProvider.shared.authentications.filter({ identifiers.contains($0.identifier) })
+        return results
+    }
+
+}

MastodonIntent/Model/Account+Fetch.swift

@@ -34,15 +34,3 @@ extension Account {
     }
 
 }
-
-extension Array where Element == Account {
-    @MainActor
-    func mastodonAuthentication() throws -> [MastodonAuthentication] {
-        let identifiers = self
-            .compactMap { $0.identifier }
-            .compactMap { UUID(uuidString: $0) }
-        let results = AuthenticationServiceProvider.shared.authentications.filter({ identifiers.contains($0.identifier) })
-        return results
-    }
-    
-}