Move session notes from filesystem to database

[lwhite1]

Problem

Session notes are currently written to text files on the local filesystem via write_task_notes() in layouts.py (line 331). Each note is appended to a file at <local_data_dir>/<subject_id>/<subject_id>-<task_name>-notes.txt.

This has several drawbacks:

• Notes are only on the CTR machine's local disk -- not accessible from other machines or the database
• Notes are not queryable or searchable
• Notes can be lost if the local data directory is cleaned up or the machine is reimaged
• No association with the log_task or log_session tables, so notes can't be correlated with other session data

Current flow

1. GUI fires _save_notes_ event (gui.py:448) or auto-saves empty notes on task completion (gui.py:523)
2. SessionController.save_notes() (session_controller.py:535) calls write_task_notes()
3. write_task_notes() (layouts.py:331) appends timestamped text to a .txt file

Proposed change

Store notes in a database table (e.g. log_task_notes) with columns for subject_id, staff_id, task_name, session_id, timestamp, and note text. Update save_notes() to write to the database instead of (or in addition to) the filesystem.