Add blade directive to know if account is linked

• add blade directive hasDiscordLinked and hasNotDiscordLinked

Examples

     @plugin('discord-auth') {{-- if plugin discord-auth is enabled --}}
           @guest  {{-- if user is not authenticated --}}
                 <a href="{{ route('discord-auth.login') }}">Connect with discord</a>
          @endguest
    @endplugin

     @plugin('discord-auth') {{-- if plugin discord-auth is enabled --}}
           @auth  {{-- if user is authenticated --}}
                @hasNotDiscordLinked {{-- if user hasn't a discord account linked --}}
                     <a href="{{ route('discord-auth.login') }}">Link your account with Discord</a>
                @endhasNotDiscordLinked
           @endauth
    @endplugin