Fix invalid names being used for persistent store

Tusker/CoreData/MastodonCachePersistentStore.swift

@@ -53,7 +53,7 @@ class MastodonCachePersistentStore: NSPersistentContainer {
             storeDescription.type = NSInMemoryStoreType
             persistentStoreDescriptions = [storeDescription]
         } else {
-            super.init(name: "\(accountInfo!.id)_cache", managedObjectModel: MastodonCachePersistentStore.managedObjectModel)
+            super.init(name: "\(accountInfo!.persistenceKey)_cache", managedObjectModel: MastodonCachePersistentStore.managedObjectModel)
         }
         
         loadPersistentStores { (description, error) in

Tusker/LocalData.swift

@@ -208,6 +208,12 @@ extension LocalData {
             self.accessToken = accessToken
         }
         
+        /// A filename-safe string for this account
+        var persistenceKey: String {
+            // slashes are not allowed in the persistent store coordinator name
+            id.replacingOccurrences(of: "/", with: "_")
+        }
+        
         func hash(into hasher: inout Hasher) {
             hasher.combine(id)
         }