Zero pad discNumber and discTotal in templates like track_number

[eharris]

Changed to zero-pad to two digits disc number template variables %M and %N just like track number %t.

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[eharris]

Is there anything more needed to get this merged?

[MerlijnWajer]

The change itself is trivial enough, I'm wondering if this change is also something that other users want. I think it could be useful, but I'm not sure. Let me ask in the IRC channel and see what others think.

[pieqq]

I feel this is a little bit of an overkill, as most releases only have 1 or 2 discs attached (but they usually have 10+ tracks, and that's why it makes sense to zero-pad these).