[ADD] slides gallery module

- Task: https://www.odoo.com/web?#id=9093&view_type=form&model=project.task
- Pad: https://pad.odoo.com/p/r.QlQTvBv6mCcMrK4B
If the goal of this field is to make an indexable content of the slides, then use `index=True` to enable PostgreSQL indexing of the column!