Perform content validity checks on main queue.

Using global background queue was handing the character counting work to a different thread every time the text changed. Forcing it always onto the same background queue would be another option, but main seems to feel fast enough and is safer because of all the view-related things downstream.

Contributes to #705 IOS client consistently hangs when writing new post at ~= 150 chars

MastodonSDK/Sources/MastodonUI/Scene/ComposeContent/ComposeContentViewModel.swift

@@ -302,6 +302,7 @@ extension ComposeContentViewModel {
     private func bind() {
         // bind author
         $authenticationBox
+            .receive(on: DispatchQueue.main)
             .sink { [weak self] authenticationBox in
                 guard let self, let account = authenticationBox.cachedAccount else { return }
 
@@ -321,7 +322,7 @@ extension ComposeContentViewModel {
 
         // bind text
         $content
-            .receive(on: DispatchQueue.global(qos: .background))
+            .receive(on: DispatchQueue.main)
             .map { [weak self] input in
                 guard let self, let detector = try? NSDataDetector(types: NSTextCheckingResult.CheckingType.link.rawValue) else {
                     return input.count