[FIX] mail: context to read follower

As the 'allow_company_ids' are now depending on the context and impact
the access to some data, we need to provide them to some controller
(route). Json route auto-pop the context when given (0322f772)
into the environment. In the 'read_follower' route, the `check_access_rules`
raises when you want to access a document of another company (even if
this one is in your allowed companies), simply because their is no
'allowed_company_ids' key in the context. This context key is automatically
added when calling the `rpc` method in JS (as explain in session.js
of a5b6f31c).
To fix the wrong access error on followers reading, we simly have to give
an empty context when calling `_rpc` in the follower widget.

Task-1999686

closes odoo/odoo#35705

Signed-off-by: Jérome Maes (jem) <jem@openerp.com>