[REF] mrp: ir.attachment handling

Rev[0] and [1] introduced a versioning on product's attachments through an
ECO creation or through a stat button on the product form view. These revisions
added some fields on the ir attachment model: a many2one and a boolean field
with a default value, and due to concerns over the migration of databases with a
lot of ir.attachment records (like ours), we adapted the implentation in rev[2][3].

We deemed reasonnable to remove the "active" field, as adding an active field not
in the “base” module of the model could impact other modules that did not took into
consideration this field. Also, this field only had a meaning when mrp_plm is
installed, and ir.attachment is used in various places in contexts across Odoo.

So, rev [2] and [3] changed the implementation of the versioning to only link
ir.attachment to an ECO record, forgetting the functionality of archiving directly
through the product form view stat button (without an ECO).

To introduce back this behavior, we chose to create a model to handle the mrp
attachments (named “mrp.document”) which inheritS ir.attachment. This way, we keep
the behavior of ir.attachment and we do not alter the original table.

This commit moved the priority field already set on ir.attachment to mrp.document
and adds the active field that will be used in mrp_plm to archive/versioning purposes.

linked commit:
[0] https://github.com/odoo/enterprise/commit/99138b6711760a7562cb9559663aef6a4208a48f
[1] https://github.com/odoo/enterprise/commit/4f88eb409776c1c4bc8f8d71d845b913c3604a54

[2] https://github.com/odoo/enterprise/commit/7e1c73cfc84cd23b6ba87dab713c3078de16568d
[3] https://github.com/odoo/enterprise/commit/ee1c4b29b8872dac9c525fcc6acdf80b5ef73131