Only serve attachments when linked to issue/release and if accessible by user (#9340)

Only serve attachments when linked to issue/release and if accessible by user (#9340)

* test: add current attachement responses

* refactor: check if attachement is linked and accessible by user

* chore: clean TODO

* fix: typo attachement -> attachment

* revert un-needed go.sum change

* refactor: move models logic to models

* fix TestCreateIssueAttachment which was wrongly successful

* fix unit tests with unittype added

* fix unit tests with changes

* use a valid uuid format for pgsql int. test

* test: add unit test TestLinkedRepository

* refactor: allow uploader to access unlinked attachement

* add missing blank line

* refactor: move to a separate function repo.GetAttachment

* typo

* test: remove err test return

* refactor: use repo perm for access checking generally + 404 for all reject