Zulip's modal_link markdown feature has not been used since 2017; it
was a hack used for a 2013-era tutorial feature and was never used
outside that use case.
Unfortunately, it's sloppy implementation was exposed in the markdown
processor for all users, not just the tutorial use case.
More importantly, it was buggy, in that it did not validate the link
using the standard validation approach used by our other code
interacting with links.
The right solution is simply to remove it.
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