[FW][FIX] calendar: allow internal users to download invitation.ics

[fw-bot]

Access rights on ir.attachment depend on the record it is linked to.

steps to reproduce:

• log as admin
• create a calendar event and invite marc demo
• log as marc demo
• check discuss notifications and try to download "invite.ics"

before this commit:

• file can not be downloaded from the webclient (access error appear in logs)

after this commit:

• file can be downloaded from the webclient

opw-3754798

---

I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

Forward-Port-Of: #160811

[robodoo]

.

[fw-bot]

@nda-odoo @arj-odoo cherrypicking of pull request #160811 failed.

stdout:

Auto-merging addons/calendar/models/calendar_attendee.py
Auto-merging addons/calendar/tests/test_calendar.py
CONFLICT (content): Merge conflict in addons/calendar/tests/test_calendar.py

stderr:

17:16:14.861646 git.c:463               trace: built-in: git cherry-pick f87b647ce5243c776dc1de3196bc7b94f7d8410b
error: could not apply f87b647ce524... [FIX] calendar: allow internal users to download invitation.ics
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
----------
status:

Either perform the forward-port manually (and push to this branch, proceeding as usual) or close this PR (maybe?).

In the former case, you may want to edit this PR message as well.

⚠️ after resolving this conflict, you will need to merge it via @robodoo.

More info at https://github.com/odoo/odoo/wiki/Mergebot#forward-port

[nda-odoo]

@robodoo r+