Add Per-Document Paste and Copy/Cut Reducers to writing_observer

[saminur]

We need to add two new reducers to the writing_observer module to capture clipboard activity at the Google Doc level with minimal changes to the existing pipeline.

The goal is to support per-document extraction of:

• paste event counts
• paste lengths
• copy counts
• cut counts
• simple clipboard summaries per student per document

[saminur]

Please take a look at the following 745ba3b @bradley-erickson @DrLynch