Added dateOnly argument to relative_modified_date

Improved the template function relative_modified_date by adding an
optional dateOnly argument which will output "today" or "yesterday" or
"x days ago".
Removed stray debug log write
Added unit tests for relative_modified_date, changed method signature

Changed method signature of relative_modified_date template method to
make it possible to add a fromTime to compare with, mostly to make it
possible to test it.

Added unit test for date and time cases.
Fixed missing default values

Added default value for $fromTime to prevent missing argument errors and
keep backward compatible.
Removed $fromTime argument from public template API
Merge pull request #5087 from owncloud/templatefunction-relativedate

Added dateOnly argument to relative_modified_date