bugfix: model: Avoid KeyError when reading user settings at startup.

[mubarak-mustopha]

What does this PR do and why?

This PR fixes #1600 .
The application crashes at startup when initializing a Model object. The code tries to read some user settings directly from initial_data, which contains the payload returned by the Zulip API POST /register endpoint.

In Zulip 12.0 (feature level 439), many of these settings were removed from the top level object into the user_settings dict.

This PR updates the code to retrieve user settings from the user_settings object when it is present in initial_data.

The tests were also updated to include all required user settings under the user_settings key when it is present in initial_data.

External discussion & connections

• Discussed in #zulip-terminal in crashes after trying to start for the first time
• [x ] Fully fixes #
• Partially fixes issue #
• Builds upon previous unmerged work in PR #
• Is a follow-up to work in PR #
• Requires merge of PR #
• Merge will enable work on #

How did you test this?

• Manually - Behavioral changes
• Manually - Visual changes
• Adapting existing automated tests
• Adding automated tests for new behavior (or missing tests)
• Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• It is a minimal coherent idea
• It has a commit summary following the documented style (title & body)
• It has a commit summary describing the motivation and reasoning for the change
• It individually passes linting and tests
• It contains test additions for any new behavior
• It flows clearly from a previous branch commit, and/or prepares for the next commit