[IMP] Delete cached JSVars on dev server startup

We need this because possibly the BaseURL has been modified and thus
the cached urls are not valid anymore. Also make
WPTPLConferenceDisplay._Types a classproperty to avoid accessing the
config path until it's accessed for the first time.
[FIX] android content disposition hack

contained a logic error and worked the wrong way round
[REF] Get rid of send_file's ftype arg
[REF] make inline=True default in send_file()