[12.0] maintenance_plan and muticompany scheme issue

[dalonsod]

Hello,

I've got a v12 installation of maintenance_plan prior to 12.0.2.3.0. This version adds some new features for plans, like a specific view and menu, which shows plans together.

Plans still haven't company_id field, so in this view you can see plans linked to equipments of other companies, and a permission error is raised when accessing to it. And equipment is not a required field for plans.

For this situation, which is the more suitable solution, adding a domain filter for thar view in order to prevent accessing to "forbidden" plans, or directly adding company_id field for plans?

cc @etobella @AdriaGForgeFlow

[dalonsod]

I've added a multi-company global rule for plans, pointing to equipment company, and it seems to work. If that's the proper solution, I'll make a PR

[dalonsod]

Implemented in #139