beautify-schedule script should output handles appropriately formatted for Zulip

[AlidaOdendaal]

The markdown-agents.txt and markdown-onboarding.txt files produced by the beautify-schedule script currently addresses agents by @<github-handle>. However, this is not the format used to ping people in Zulip. The format recognized by Zulip is @**<Full name>**, where the choice of Full name depends on the person (and can change at any time), and may or may not be equivalent to the person's Github handle.

The current workaround is to paste the markdown file contents into Zulip, and to "Tab" through the handles so that Zulip can autocomplete them to the correct form required for the pings.

The best way to automate this may be to ask the team to make sure that the Github handle field under their Personal settings in Zulip is set to their actual Github handle. Then, this script could connect to the Zulip API to read both the Github handles and Full names, and thus output the correct @**<Full name>** pings to the markdown-agents.txt and markdown-onboarding.txt files.