Set email asset host dynamically

Is your feature request related to a problem? Please describe.

It can be strange for users to find resource URLs that are linked to another organization, this development allows to either define a common asset host for all organizations (example: assets.decidim.org) or to reroute assets according to the organization's host.

Describe the solution you'd like

I would like to set email asset host dynamically if the application asset host is empty.

Describe alternatives you've considered

A workaround is to set an asset host to a neutral url like "assets.decidim.org"