Switch over from GCM to FCM

[ayushin]

My understanding that nobody uses GCM anymore, so perhaps it is time for the upgrade?

My suggestion would be to rename GCM model to FCM model (breaking change) and add platform choices/enum to it, like I've done in the #615

I agree perhaps there is no reason to keep the GCM model anymore and message_type but it will be a (small) breaking change.

What are your thoughts?

1. Rename GCM to FCM?

2. Keep GCM as a proxy model for backward compatibility? (seems like unnecessary work)

3. platform choices:

a) android, ios, web - full text in the database consistent with the rest of the library
b) a, i, w - save space
c) 0, 1, 2 - save space, easy indexing, sorting
d) new django enums