Improve onboarding_steps subystem.

[prakhar1144]

Follow-ups of #29296 & improvements for onboarding_steps subsystem as a whole:

• Rename test_hotspots.py to test_onboarding_steps.py
• Remove tutorial_status field from UserProfile model. -- Fields is no longer used.
• Remove onboarding_status field from UserProfile model. -- Fields is no longer used. (b5e22bf)
• Fix ui_init.js to not store onboarding_steps state data as current_user_params entry & related changes.
• Update https://zulip.readthedocs.io/en/latest/subsystems/hotspots.html

[zulipbot]

Hello @zulip/server-onboarding members, this issue was labeled with the "area: onboarding" label, so you may want to check it out!