allow empty configvalue in appconfig

Use plural translations

Send mime-type for new files

Fix sharing defaults

Fix recursive scanner to reusing etags

Do not pass numeric strings to strtotime() when formatting dates.

strtotime() will return false and the date will be formatted as the 0-timestamp
(e.g. January 1, 1970 00:00) otherwise.

show 'More apps' link to app administration directly in app navigation