Allow browsers internal spellchecker to work

Fix a regression introduced at 731d190 that prevents in-browser spellchecker from working.
The new TinyMCE4 option is "browser_spellcheck", before it was "gecko_spellcheck".

Related ticket: http://trac.roundcube.net/ticket/1485060
Remove $config['default_folders'] from config tips

Remove $config['default_folders'] from config tips as this option was removed on commit dc0b500.
Related ticket: http://trac.roundcube.net/ticket/1489737
Set edited folder name to client environment