Markdown in flashes removing HTML links

[orenyk]

Since we started Markdown-rendering our flashes (#1027, my bad) we've been removing all HTML in our flash messages since our Markdown renderer was configured to do so. As a result, the persistent view mode flash no longer has functioning links.

This can be easily solved by a) not removing HTML during Markdown rendering or b) optionally not removing HTML during Markdown rendering by passing a second parameter to the markdown method (in ApplicationHelper and in _flash_alerts.html.erb`). I vote for the latter, this should be a quick fix and we can stick it in v5.2.0.