Fixed group header date format

- Header date format is now based on date only (not time) which will
  show relative date texts like "Today" or "Yesterday".
- Moved strip_time to owncloud core's template functions
Removed unused "Today" translation key

"Today" is now being translated when using relative_modified_date from
core's template functions.
Now grouping activities by header name

From now on, the activities are not grouped by single days but by
grouping string, like "Today", "Yesterday", "2 months ago", etc.
Now using sanitizeHTML() instead of htmlspecialchars
Fixed warning about passing references to sanitizeHTML()

It seems that sanitizeHTML() requires a variable to be passed else PHP
will show a warning in the log.

This commit moves the relevant values into variables first.
Now caling translation function for JS messages
Merge pull request #1394 from owncloud/activity-groupheaderdateformat

[Activity] Fixed group header date format