purple: feature flag management

Originally the Damus Purple feature was gated behind a setting which
served as a feature flag.

However, when we release, we need to enable Purple features for all
users running on a particular version.

This commit improves feature flag management by replacing the setting
with a computed property (which still points to the setting, but can be
very easily replaced)

Signed-off-by: Daniel D’Aquino <daniel@daquino.me>
Reviewed-by: William Casarin <jb55@jb55.com>
Signed-off-by: William Casarin <jb55@jb55.com>

damus/Views/Profile/ProfileName.swift

@@ -113,7 +113,7 @@ struct ProfileName: View {
         }
         .onAppear(perform: {
             Task {
-                 if damus_state.settings.enable_experimental_purple_api {
+                 if damus_state.purple.enable_purple {
                     self.purple_badge = await damus_state.purple.profile_purple_badge_info(pubkey: pubkey)
                  }
              }