Fixes #3435: Some labels of module 'project' are now translatable

The labels of fields `use_tasks`, `partner_id`, `date_start` and the project visibility labels (of module `project`) are now translatable.

Note that there's one more visibility setting in `portal_project` which isn't included in this pull request as "A bugfix must not require updating the source code of 2 different modules at the same time"