[FIX] project: restore portal visibility

Odoo 11 comes with unified features for Project Tasks and Project
Issues. Specifically, issues were retro-fitted into tasks, and saw their
specific features added to the features of tasks.
During this refactoring, the portal controller for issues was superseded
by a unified portal controller for tasks, via
623ea432681a8a0861fec6db0194f3911429efb2

One case was lost in the process: portal visibility of
tasks/issues/things that are individually followed by customers, without
the customer following the project itself.
This is important for `transversal` projects, such as a "Helpdesk"
project common to all customers, where each customer only sees their own
issues.

This commit restores the feature, previously added to issues by
1cc8252a953ba6adeeb5dead2c82b336cd51c9db

Note: this clearly requires a proper regression test, to be added soon.

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