MessageView: fix crash when clicking on URLs in WebView

The ContextThemeWrapper added in a74d57cb
used getActivity().getApplicationContext() to get the base context.
This is wrong, because an Application context won't work for starting
activities from the WebView.

Instead, use the context that is given to us in the constructor as base
context. This is the one that would also  be used if no ContextThemeWrapper
was present at all.