Restrict size of text fields in event pop-up

This small styling fix prevents the description of an event
to grow uncontrollably in either width (e.g. in the
presence of a long URL that otherwise cannot go to a
newline), or height (by introducing a maximum relative to
the viewport, and scrollbar otherwise).

Signed-off-by: Matteo Settenvini <matteo.settenvini@montecristosoftware.eu>