Update EMM references to new name

[mjangda]

Before publicly releasing, we should try and update all references to extended-media-manager or emm to media-explorer or mexp_ to match the new name.

[pepegar]

@mjangda, I've changed the naming of classes and functions containing EMM or extended media manager in f446167.

extended-media-manager --> media-explorer
emm --> me

Let me know if you like the names I've chose for them so I can submit a PR and merge.

[johnbillion]

I wonder if mexp might be a better prefix than me?

[johnbillion]

The textdomains will also need to change from emm.

[simonwheatley]

I wonder if mexp might be a better prefix than me?

It's more likely to be unique, I'd guess; me is a very tempting prefix. :)

[pepegar]

Yes, totally agree @johnbillion @simonwheatley, I'm changing it. Thanks for the feedback!

[pepegar]

Pushed some more changes in fe424e8 following the advices of @simonwheatley and @johnbillion.