Ingest archived Tigris SVN repositories

Dumps of all known SVN repositories on Tigris shortly before the platform's shutdown on 2020-07-01 are available on archive.org.

We should ingest those repositories into the archive through the following actions:

• Update implementation of swh.loader.svn.loader.SvnLoaderFromRemoteDump to take the path (local or remote) of a SVN repository dump file as a constructor optional parameter, and use this file to recreate the repository instead of calling svnrdump dump command.
• [anlambert] Update implementation of SvnLoaderFromDumpArchive to support downloading a dump file prior mounting it, and resinstate scheduler task for it: swh/devel/swh-loader-svn!253
• [anlambert] Tag loader-svn v2.3.0
• [sysadm] Deploy the v2.3.0 version of the loader-svn
• [sysadm] Test with a subset of the projects in staging
• [sysadm] Schedule loading tasks manually using a command like:

$ swh scheduler task add -p oneshot load-svn-from-archive url=http://${project}.tigris.org/svn/${project} archive_path=https://archive.org/download/tigris_svnrdump_202006/{project}.svnrdump.gz