[3.0] Fixes two issues in SMF\Actions\TopicRestore

[Sesquipedalian]

1. Uses correct JOIN syntax in some calls to User::loadCustom()
   • In SMF\Actions\TopicRestore::mergePosts(), a call was made to SMF\User::loadCustom() in which the 'joins' element of the 'query_customizations' argument wasn't a complete JOIN statement. This causes an SQL syntax error. A similar issue was found in SMF\Actions\Admin\Subscriptions::modify().
   • For both cases, this commit fixes the malformed JOIN statements.
2. Casts to correct types when calling Actions\TopicRestore::mergePosts()
   • In SMF\Actions\TopicRestore::execute(), the mergePosts() method was being passed a string rather than an integer, causing an error to occur.
   • This has been fixed by casting the argument to int.