As a user, I can define multiple test natures in a given test issue

Users must be able to define more than one test type for each Test issue. Currently, when a test type changes, the current definition is deleted. Xray must provide a way to keep different implementations for the same test case, one for each test type.

Users should be able to add new test natures and see all test natures in different tabs on the test details web panel. Only a single definition must be the default at each time. When Test Runs are created, the default definition is used.