assets/save: Add multi artifacts support for archives visit type

Enhance save code now Web UI for the archives visit type: multiple
artifacts can now be submitted to be loaded under the same origin.

User can now easily add a new form row to fill new artifact info
through a dedicated button.

New form rows can also be removed by dedicated buttons.

Add an help text above the artifacts form to explain how source
code will be loaded into the archive.

Related to T3213