more alterations Settings and Mod related (#793, #216)

- correct date/time format descriptions, add hint to php.net
- add hint how to give custom css file path
- removed obsolete warnings
- add own descriptions for Venue page settings
- mod_jem: use own date/time format (JemOutput::formatDateTime enhanced too)
- fixed default to "%H:%M" (colon)
- Wide/Teaser mod: reformatted tmpl to prevent unneeded blanks (which may cause linebreak e.g. between time icon and time)