Change default rails port

[andreslucena]

While working on development, we're using the port 8000 by default. As @alecslupu mentioned in a review:

I already had something else binding 8000, I had to reconfigure the bulletin board ports here and in the secrets.yml.

As 8000 is a pretty popular port, maybe would be better to document the workaround ?

We need to define one and change it in the default configurations of this repository and also of decidim/decidim:

• In puma.rb
• In secrets.yml template of decidim/decidim

Now we're using port 5017 for the testing instance, so 5016 might be a good number, as this range isn't assigned in the IANA:

5016-5019: Unassigned

And also doesn't seem to be popular for other services. As an exercise, compare it with 8000.

Acceptance criteria

• Port changed in puma.rb
• Port changed in secrets.yml template in decidim/decidim
• Port changed in documentation/READMEs/etc