Fix channel message pinning for channel moderators

The way of fixing this is weird and is a natural clutch. But we have to
live with it. And I hate it.

Note: all another movings are postponed to another commit.

This commit based on
telegramdesktop/tdesktop@75d8d01#diff-efcc341e70654031bd8fee83d8b7105bR999

Fixes #189.

Telegram/SourceFiles/structs.cpp

@@ -922,6 +922,15 @@ void ChannelData::setPinnedMessageId(MsgId messageId) {
 	}
 }
 
+bool ChannelData::canPinMessages() const {
+	if (isMegagroup()) {
+		return (adminRights() & AdminRight::f_pin_messages)
+			|| amCreator();
+	}
+	return (adminRights() & AdminRight::f_edit_messages)
+		|| amCreator();
+}
+
 bool ChannelData::canNotEditLastAdmin(not_null<UserData *> user) const {
 	if (mgInfo) {
 		auto i = mgInfo->lastAdmins.constFind(user);

Telegram/SourceFiles/structs.h

@@ -909,9 +909,6 @@ class ChannelData : public PeerData {
 	bool canAddAdmins() const {
 		return adminRights().is_add_admins() || amCreator();
 	}
-	bool canPinMessages() const {
-		return adminRights().is_pin_messages() || amCreator();
-	}
 	bool canPublish() const {
 		return adminRights().is_post_messages() || amCreator();
 	}