Fix custom instances not showing up when typed into the instance

selector search field

With Combine Cancellables are automatically cancelled on deinit, so the
instance selector needs to hold on to a reference to pipeline
cancellable for its lifetime, otherwise it's cancelled immediately after
creation.

Closes #59

Tusker/Screens/Onboarding/InstanceSelectorTableViewController.swift

@@ -26,6 +26,7 @@ class InstanceSelectorTableViewController: UITableViewController {
     var recommendedInstances: [InstanceSelector.Instance] = []
     
     let urlCheckerSubject = PassthroughSubject<String?, Never>()
+    var urlHandler: AnyCancellable?
     var currentQuery: String?
     
     init() {
@@ -66,7 +67,7 @@ class InstanceSelectorTableViewController: UITableViewController {
         navigationItem.searchController = searchController
         definesPresentationContext = true
         
-        _ = urlCheckerSubject
+        urlHandler = urlCheckerSubject
             .debounce(for: .seconds(1), scheduler: RunLoop.main)
             .compactMap { $0?.trimmingCharacters(in: .whitespacesAndNewlines) }
             .sink(receiveValue: updateSpecificInstance)