[emacs] .dir-locals.el: use jinja2-mode to format html files

The jinja2-mode [1] can be installed from melpa [2]::

    M-x package-install / jinja2-mode

[1] https://github.com/paradoxxxzero/jinja2-mode
[2] https://melpa.org/#/jinja2-mode



Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>