Consistently use `.j2` file extension for all Jinja2 templates

We should use the .j2 file extension for all Jinja2 templates. This allows will allow to use proper/correct code highlighting in modern IDEs. For developers using battleproofed IDEs, configuration options do exist, e.g. [1][2][3].

The current state is that templates sometimes have the suffix and sometimes not. This means that both sides (modern IDEs as well as battleproofed w/o proper configuration) do frequently suffer from faulty code highlighting. Therefore, the file extension needs to be made consistent.

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• [1] https://unix.stackexchange.com/questions/137989/how-to-change-default-syntax-highlighting-for-header-file-in-kate
• [2] https://github.com/lepture/vim-jinja
• [3] https://github.com/paradoxxxzero/jinja2-mode