Clean up discard duplication in IDE

What does this MR do?

The actions discardAllChanges and discardFileChanges contained some serious duplication and were also missing some functionality between the two. This MR consolidates the shared bit into another action.

Screenshots

Since the duplication across the two actions wasn't perfect, the "Discard all" feature was missing a step where the parent directory is restored when restoring deleted files.

Before	After
20200113_discard_duplication_bug	20200113_discard_duplication_fix

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

References

• #31859 (closed)