WIP - Allow saving of a broken question

[EJMFarrow]

• Adds an isbroken flag to STACK questions. This can be set using a tick box at the bottom of the edit page. It allows a STACK question to be saved even if STACK-specific validation fails.
• Save does not occur if the user needs to confirm removal of inputs/PRTs. A warning is displayed and the user must check the confirmation boxes.
• Save does not occur if there are Moodle validation errors. This will show a 'not saved' warning in most cases but not if the only problem is missing required fields - that's a browser-side check.
• Fixed existing bug with non-display of Moodle validation errors (e.g. set default mark to -1).
• Save does occur if the user goes straight for Save rather than `Verify and update form' (even when a new PRT is being added).
• Validation messages for a broken question are displayed on loading/reloading of the edit page.
• Requires a DB update for the new field.
• Broken questions throw a runtime error so are not displayed to students or on preview, etc.
• Restore, backup, export, import updated.
• Behat tests added.
• Added search for questions where latest version is broken to dependencies page.

This is at the stage where it could do with review. What have I missed?

[EJMFarrow]

Updated to display the not-saved warning when validation fails normally (i.e. when the question is not marked as broken).

[sangwinc]

Thanks @EJMFarrow