Add timestamps to activities and assets declarations

[raul-fuentes]

References

• Añadir fecha de creación y actualización de las declaraciones #171

Objectives

• Added columns created_at and updated_at to activities_declarations and assets_declarations, can be nil and are automatically filled

Visual Changes (if any)

• None

Notes

• none

[coveralls]

Coverage increased (+0.1%) to 91.498% when pulling 9073f91c2af8376c70d89bd930249342260a80cd on wairbut-m2c:add_timestamps_to_activities_and_assets_declarations into 9d472330dcbdb5ad2c7a580f6949ef485d2505f2 on AyuntamientoMadrid:master.

[javierm]

Closed in favor of #195. I didn't realize this one already existed.