filter filetypes for latest sermon view

[ghost]

Hi Thomas,

I'm using the "latest sermon" menu option and found, that the filetype filter does not work in this case.
I want to link to the latest sermon, that also has an audio file attached (ignore sermons without audio).
I enabled the option with this fix. Now I can set the filetype filter to 'audio' and get what I want.
I thought you might want to pull this so I prepared a pull request for you.

Grüße
Frieder

[Bakual]

I think this needs a bit of improvement so it uses states instead of directly reading the params. Other than that it looks fine to me. Also need some time to test it.

[ghost]

I tried to implement it using states first but calling getState() inside getLatest() causes populateState() to be called too early (when id is not yet set). This is why I read from the params directly.
But I'm far from being an expert, so there might be better ways to do this.

[Bakual]

I've cherrypicked your commit and improved it so it uses states now. Also did some other improvements on the way.

Thanks!