Plugin Directory: Short Descriptions shouldn't display unrendered Markdown

[webdevmattcrom]

Currently, if a plugin has Markdown in its short description, that is shown unrendered in the search result tiles. The "Give" plugin is a good example in these results:
​https://wordpress.org/plugins/search/donations/

It seems this changeset was intended to resolve that, but perhaps that only applied to the single plugin pages and not search results:
https://meta.trac.wordpress.org/changeset/3360

[dd32]

In 5205:

Plugin Directory: When the Short description isn't provided, pull it from the main plugin description after markdown has been parsed - this allows the parser to strip out unwanted markup in the description.

Fixes #2643