Update task due notification template

* For each task past due, add link to task on kanboard if application_url is specified.
* Add task due date.
* Remove task link on footer.
Update helpers.php, function a()

In helpers.php is a function a() used for link generation, i. e. by markdown(). In the opening <a> element was a /, which is ignored by many (all? most?) browsers, but leads to interesting result with jQuery inserting of HTML generated by that function.

I stumbled upon this while developing code for a preview of textareas and not being able to show a link to a task (#1).
Merge pull-request #356 (bugfix Helper\a())
Add Thai translation (pull-request #353)