Improve app_offline.htm behavior to ensure successful builds

[ankitkumarr]

app_offline.htm gets created during a kudusync copy and some times, it may fail to remove. We need to discuss and see if there's a better approach.

cc: @suwatch

[suwatch]

Thanks for tracking this! Let's brainstorm some idea this week.

[ankitkumarr]

Next steps --

1. Find evidence of cases where this happened
2. Set up Alerts for this
3. Finalize on the design.
   One of Suwat's suggestion was going with a two-phase commit, where the first step involves dropping a file somewhere outside of wwwroot, indicating that a app_offline.htm may be created. Periodically, kudu could check for this file and if present, remove app_offline.htm, keeping in mind any ongoing deployments.