Fix exceptions there isn't an organization with a host

[andreslucena]

🎩 What? Why?

When there is a host (i.e. a domain or subdomain) pointing to Decidim but there isn't an organization with that host (i.e. when there is a proxy misconfiguration) the application is giving an Exception instead of redirecting to System login (as it should do).

When fixing this exception then you have another exception 😅

This PR fixes these Exceptions.

Testing

1. Add a host in your host file (/etc/hosts in GNU/Linux). For instance 127.0.1.1 localhost999
2. Add the host to you config/application.rb: ` config.hosts << "localhost999"
3. Restart your app server
4. Visit http://localhost999:3000/processes with a browser (Mind that you can't have the exceptions if you go to http://localhost999:3000/)
5. See the exception (without the patch)
6. Apply the patch
7. See that the redirection works

Stacktrace

First exception (UseOrganizationTimeZone)

NoMethodError (undefined method `time_zone' for nil:NilClass):

home/apereira/Work/decidim/decidim/decidim-core/app/controllers/concerns/decidim/use_organization_time_zone.rb:28:in `organization_time_zone'
home/apereira/Work/decidim/decidim/decidim-core/app/controllers/concerns/decidim/use_organization_time_zone.rb:21:in `use_organization_time_zone'

Second exception (ForceAuthentication)

NoMethodError (undefined method `force_users_to_authenticate_before_access_organization' for nil:NilClass):

/home/apereira/Work/decidim/decidim/decidim-core/app/controllers/concerns/decidim/force_authentication.rb:20:in `ensure_authenticated!'

♥️ Thank you!

[alecslupu]

LGTM