offer MYSQL_COMPATIBILITY setting to remove unique from the model

[tuky]

Fixes #266. Funny enough I just saw the commit with the swappable model and it seems possible to do this now:

class CustomFCMDevice(AbstractFCMDevice):
    registration_id = models.CharField(  # https://github.com/xtrinch/fcm-django/issues/266
        verbose_name="Registration token",
        unique=True,
        max_length=600,  # https://stackoverflow.com/a/64902685 better to be safe than sorry
    )

    class Meta(OriginalFCMDevice.Meta):
        pass

with setting FCM_DJANGO_FCMDEVICE_MODEL = '...CustomFCMDevice'.

[xtrinch]

Should make it clear somehow this is only an edge case and most people even if using mysql won't need this (and mention what the use case would be)

[tuky]

improved it and explained my alternative solution that even lets mysql users enjoy an indexed registration_id

[xtrinch]

Why explicitly check this? It should skip it if it's mysql anway?

[tuky]

To make makemigrations happy. After adding

FCM_DJANGO_SETTINGS = {
    'MYSQL_COMPATIBILITY': True,
}

to the default test settings:

1st execution of makemigrations is with the check.
2nd execution is without it.

[tuky]

btw, i would be happy with only merging the readme part regarding a custom model and ditch the code & setting part. for us the custom model is enough as soon as it's released.

[xtrinch]

I wouldn't want everyone to have to migrate to custom model because of this and there seems to be a lot of people having issues with this.

[xtrinch]

Fixes #266, #258