Support previewing as another user

**Is your feature request related to a problem? Please describe.**

After setting up a session for a module, it would be good to preview what students will see. Similarly, if a student is facing a problem, it would be good if we can simulate what that particular user sees.

**Describe the solution you'd like**
Provide a way to preview as another user. e.g., by adding `?user=adamGan` to the URL.

Things to consider:
1. In this mode, all write operations can be (should be?) disabled. Some of them will not work anyway.
2. Keep the feature hidden? That is, for the use of devs only, not mentioned in the UG.
3. Should the current user (e.g., the dev whos is using CATcher for troubleshooting a problem) be in the data.csv file? It is possible to add all devs as a separate team in the data.csv.

Start by locating the URL query handling, session/user data flow, and write-operation paths. Resolve the issue's open questions about disabling writes, keeping the mode hidden, and including developers in data.csv with maintainers. Done means a defined, tested way to preview another user's view without unsafe changes.

Written by the indexing model from the issue text.