Fix pluralizations of models on admin page

[betaveros]

Kind of related to #1321, but at least we should tell Django that (for example) the plural of "category" is "categories". I don't think Meta class changes would require database migrations or anything of that sort? Basically just do this: https://docs.djangoproject.com/en/1.8/ref/models/options/#verbose-name-plural

[betaveros]

Oh I guess I should link #2357 to this.

(I wouldn't mind leaving these pluralizations as tiny changes for people to learn how to contribute with, but I think I can find more minor issues eventually.)

[Nickatak]

Hello! Is this still an existing issue?

[milescalabresi]

Yes, please feel free to address it if you'd like. I dropped the ball

[Nickatak]

Ofcourse. I'm stuck on something else at the moment, so I will start this tomorrow when I have time (I also need time to read through the docs/readme's to make sure I'm doing this right, as I'm pretty new). :D

[Nickatak]

@milescalabresi Do you happen to have a list of which ones are/are not completed? Or would you prefer me to just go through each one and check?

[milescalabresi]

I don't, sorry. I remember them being pretty easy to notice, though.

[Nickatak]

Erm, I understand what must be done here, but I have no good way to test this (I have a win box only right now). I spent the last day or so trying to get this to run, but I think I'm going to have to decline this due to not having a proper testing environment available for it, sorry :C