Applying filters on an activity module description when using it as a
new calendar event's description is bad m'kay? We need to store the raw
text and apply the filters only when we actually display the text. That
way, filters (such as multi-language content) may actually fully work
and we do not initialise the theme and output machinery.

Additionally, we need to explicitly set the format of the description
text to HTML (because we have converted it to it already). Otherwise it
defaults to the current user's preferred editor format.

This is still a pragmatic hot-fix solution. The proper solution would be
to pass the raw text, format and embedded files.