Default task list API

[opatry]

Description

• There is a special Task List ID @default that can be used to retrieve the default task list.
• Provide the TaskListsApi.default() API to resolve it
• Update data model to allow querying isDefault needed to implement TaskListUIModel.canDelete
• As a quick & dirty workaround, the isDefault is currently inferred from ID size, an upcoming PR should come to implement the remote default API resolution properly.

Checklist

• I have read the CONTRIBUTING guide
• Code compiles correctly
• Created tests which fail without the change (if possible)
• All tests passing