As a security precaution, we change the contract of some signals such that a
SafeString needs to be returned if HTML should be rendered without further
escaping.
Before, the `{% signal ... %}` and `{% eventsignal ... %}` template tags
called mark_safe themselves on all strings returned from signals. That could
lead to unsafe coding practices, where untrusted values are interpolated into
HTML format strings. However, such interpolations should usually be performed
using helpers such Django's format_html, which automatically escapes inputs
and returns a SafeString.
Now, we call conditional_escape on signal results, so that any HTML not explicitly
marked as safe gets escaped.
Most plugins are not affected by this change as they return a SafeString as a
result of Template.render already.
