Add towncrier (#787)

* Add towncrier

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* .gitkeep -> git.keep

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Remove unused changelog-check script

check-newsfragment script is used instead

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>

* Update check-newsfragment script

- Link to the correct PR guidelines
- Filter out git.keep

Signed-off-by: Andrew Ferrazzutti <andrewf@element.io>