Todoist extension should not offer to add a Todoist account if it is not available
At the moment, the Todoist extension points the user toward creating a Todoist account in GNOME Online Accounts. However, this even happens on systems where Todoist accounts are not available, such as the current version of Fedora 27 (where gnome-online-accounts is built without the --with-todoist flag). In that case, clicking the "Add a Todoist account" does nothing useful. It opens System Settings, at the Online Accounts panel, and that is all.
Steps to reproduce:
- In Fedora 27, open GNOME Todo.
- Open the Extensions dialog, enable Todoist.
- Open the Todoist extension settings.
- Press the button that says "Add a Todoist account".
- Note the rather confusing result: GNOME System settings opens, showing nothing about Todoist.
The plugin should use libgoa to determine if Todoist is available. If not, it should fall back gracefully, perhaps with a note that Todoist support is unavailable.