streams: Make default notifications settings customizable per-stream.

[Ujjawal3]

This PR introduces a new boolean field push_notifications_enabled in Steam model. If this field is set to true push notifications will be enabled by default for new subscribers of steam. Default value of this field is false.

Fixes: #23873

Screenshots and screen captures:
ScreenShot showing Stream Creation form for owner

Tooltip shown when push notifications are not configured on server.

Self-review checklist

• Self-reviewed the changes for clarity and maintainability
  (variable names, code reuse, readability, etc.).

Communicate decisions, questions, and potential concerns.

• Explains differences from previous plans (e.g., issue description).
• Highlights technical choices and bugs encountered.
• Calls out remaining decisions and concerns.
• Automated tests verify logic where appropriate.

Individual commits are ready for review (see commit discipline).

• Each commit is a coherent idea.
• Commit message(s) explain reasoning and motivation for changes.

Completed manual review and testing of the following:

• Visual appearance of the changes.
• Responsiveness and internationalization.
• Strings and tooltips.
• End-to-end functionality of buttons, interactions and flows.
• Corner cases, error conditions, and easily imagined bugs.

[zulipbot]

Heads up @Ujjawal3, we just merged some commits that conflict with the changes you made in this pull request! You can review this repository's recent commits to see where the conflicts occur. Please rebase your feature branch against the upstream/main branch and resolve your pull request's merge conflicts accordingly.