[Report] Add report reopen action for closed reports

- New Command_ReportReopen moderator command (extension 1206) with server
  handler that reopens resolved/dismissed reports, resets resolution state
  and notified, and records the reopening note in the audit log
- Reopen... button in the report queue action bar, gated to resolved and
  dismissed reports, with optional note prompt and tooltip
- Success confirmations for Assign, Resolve/Dismiss and Reopen now survive
  the list refresh via a pending status message instead of being clobbered
- Dispatch/auth coverage in server_moderator_role_test

libcockatrice_protocol/libcockatrice/protocol/pb/command_report_reopen.proto

@@ -0,0 +1,10 @@
+syntax = "proto2";
+import "moderator_commands.proto";
+
+message Command_ReportReopen {
+    extend ModeratorCommand {
+        optional Command_ReportReopen ext = 1206;
+    }
+    required int32 report_id = 1;
+    optional string reopen_note = 2;
+}