Add sterner warning about post content type

Tusker/API/InstanceFeatures.swift

@@ -50,6 +50,10 @@ struct InstanceFeatures {
             || (instanceType == .pleroma && hasVersion(2, 0, 0))
     }
     
+    var probablySupportsMarkdown: Bool {
+        instanceType == .pleroma || instanceType == .glitch || instanceType == .hometown
+    }
+    
     mutating func update(instance: Instance, nodeInfo: NodeInfo?) {
         let ver = instance.version.lowercased()
         if ver.contains("glitch") {

Tusker/Screens/Preferences/AdvancedPrefsView.swift

@@ -23,7 +23,17 @@ struct AdvancedPrefsView : View {
     }
     
     var formattingFooter: some View {
-        Text("This option is only supported for Pleroma and Mastodon instances with formatting enabled. On all other instances, formatting symbols will remain in the unformatted plain text.").lineLimit(nil)
+        var s: AttributedString = "This option is only supported with Pleroma and some compatible Mastodon instances (such as Glitch or Hometown).\n"
+        if let account = LocalData.shared.getMostRecentAccount() {
+           let mastodonController = MastodonController.getForAccount(account)
+            // shouldn't need to load the instance here, because loading it is kicked off my the scene delegate
+            if !mastodonController.instanceFeatures.probablySupportsMarkdown {
+                var warning = AttributedString("\(account.instanceURL.host!) does not appear to support formatting. Using formatting symbols may not have an effect.")
+                warning[AttributeScopes.SwiftUIAttributes.FontAttribute.self] = .caption.bold()
+                s += warning
+            }
+        }
+        return Text(s).lineLimit(nil)
     }
     
     var formattingSection: some View {