add a "latest" link to dl.bitmask.net

Right now, gitlab CI is producing an artifact for every master merge on the following link:

https://0xacab.org/leap/bitmask-dev/builds/artifacts/master/download?job=bundle

but it downloads a file with a non-very-informative name.

Things to discuss:

1. is it possible (from gitlab) to name the file differently?
2. can we create a redirect from the downloads.bitmask.net apache? ideally, a nightlies/ folder should be created on the root public resource, with a linux-64bits.tar.gz pointing to the gitlab artifact.

(from redmine: created on 2016-11-14, closed on 2017-01-15)