Add created to group membership model

[seanh]

Add a created column to models.GroupMembership.

Fixes #9103. This should automatically mean that new group memberships get their created and updated times tracked, without any further changes.

Before we can merge this PR we'll need a migration to add the new columns.

Then after merging this PR we'll need another PR to add the created date to the responses from various APIs.

While we're doing this, might as well add updated as well.

There's a desire for this column to be nullable so that pre-existing
memberships can have NULL in this column, see:
https://hypothes-is.slack.com/archives/C4K6M7P5E/p1733330444569829

For that reason we can't use h.db.mixins.Timestamps because that mixin
adds non-nullable created and updated columns.

I don't think it really matters but if we did change the
GroupMembership model to inherit from the Timestamps mixin that
would add the created and updated columns to the start of the
table, whereas in the staging and production DBs a migration is going to
add the new columns to the end of the table. Meaning that DBs created
from the models (e.g. in dev and on CI) would actually have the columns
in a different order than staging and production. Not sure if there
would be a way to fix the column order while using mixins.Timestamps.
Moot point anyway since we need them to be nullable.

[marcospri]

Looks good 👍

We do need the migration before merging this as you say.