Simplified autoincremented integer ID migration

[florimondmanca]

Is your feature request related to a problem? Please describe.
Once #60 is solved, we should be able to provide a much simpler migration for the integer ID.

Describe the solution you'd like
Update AbstractAPIKey by dropping the custom id column, and generate a migration which simply drops the current id field and adds the autoincremented one.

As hinted in #42 (comment), adding/populating the new ID column and dropping the old ID column should be done in two separate migrations. This will allow users to take action in a state where both IDs exist.

We'll still need a migration guide for users with models referring to an APIKey model. See #48

Describe alternatives you've considered
/

Additional context
See #40 and #41 for the previous implementation

[florimondmanca]

I'd really like to get the PK migration over the line as 2.0. There's still #63 but I'm missing some time currently to implement this. If this is blocking some users, any help/PRs would be gladly appreciated. :-)

[florimondmanca]

Closing as this is now out of date. Will open another issue for PK migration.