boxes: Add edit countdown timer and modify edit view for stream and PM.

[prah23]

Added edit_countdown_timer which runs asynchronously for as long as the edit compose box is open. The timer refreshes every minute while the time remaining is over 1 minute, and every second otherwise. The stream_box_edit_view has been modified to display the timer. private_box_edit_view has been added as a separate view for editing private messages.

Fixes #727 and #752.

Screenshot:

[prah23]

@zulipbot remove "further discussion required"

[prah23]

@zulipbot remove "has conflicts"

[neiljp]

@prah23 Could you please remove the merge commit and rebase against upstream instead?

[prah23]

@neiljp Thank you for the review! I apologize for the merge commit, I am aware of Zulip being rebase-based, I noticed a couple of conflicts and was trying to resolve them and I merged them accidentally. I was in the midst of trying to sort it out and fix linting right now, while you were reviewing it. I will make the changes as discussed and push clean commits.

[zulipbot]

Heads up @prah23, we just merged some commits that conflict with the changes your 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.