Loosen the subsetting relationship between default and translation builds

Via doc-suse-com/site#49 : We should be able to enable additional document formats for certain translated documents.

This could be done by allowing a new element to allow additional formats within translation definitions, e.g. extraformat:

      <language lang="es-es" translation-type="list">
        <branch>maintenance/SLE15SP3</branch>
        <subdir>l10n/sles/es-es</subdir>
        <deliverable>
          <dc>DC-SLES-installation</dc>
          <extraformat html="1"/>
        </deliverable>
      </language>

This could be used to cut down on the massive number of pseudo-translated actually-English pages that we have on dsc currently. There are unwanted side effect of fully updating our configs to adopt this, however:

• It would mean that English and non-English URLs would often not be aligned, meaning that URL manipulation to add/change a language code would no longer work in every case.
• It would mean that a lot of our non-English pages change URL. That would probably cause a bit of an SEO ruckus.
• It would mean configuration would become longer. Oops.

However, those issues should be massively outweighed by the fact that we no longer ship all those unnecessary deadwood pages.

To be defined:

1. Would it be possible to remove formats via extraformat? In that case, it should probably be named differently.
2. Would it be necessary to re-define all formats that are already defined for the default language? Or would it be enough reference those that are changed?

[cc @jfaltenbacher -- thanks for the discussion today; this has helped me realize that this allows a way to fix a much bigger issue on our websites along with fixing how links are displayed.]