Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (trailing-whitespace)
• Use versioned copyright format URI. (unversioned-copyright-format-uri)
• debian/copyright: use spaces rather than tabs to start continuation lines. (tab-in-license-text)
• Bump debhelper from old 12 to 13. (package-uses-old-debhelper-compat-version)
• Set upstream metadata fields: Bug-Database, Bug-Submit, Repository-Browse. (upstream-metadata-file-is-missing, upstream-metadata-missing-bug-tracking)
• Update standards version to 4.6.1, no changes needed. (out-of-date-standards-version)

Debdiff

These changes affect the binary packages:

[The following lists of changes regard files as different if they have different names, permissions or owners.]

Files in second set of .debs but not in first

-rw-r--r--  root/root   /usr/lib/debug/.build-id/93/5c17ca22de79016f9d0c37db313c76b461d63b.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/5e/e4935e5eaf54d9c9d028b90f3d42b5982a0b26.debug

No differences were encountered between the control files of package **xdrawchem**

Control files of package xdrawchem-dbgsym: lines which differ (wdiff format)

• Build-Ids: 5ee4935e5eaf54d9c9d028b90f3d42b5982a0b26 935c17ca22de79016f9d0c37db313c76b461d63b

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Build and test logs for this branch can be found at https://janitor.debian.net/lintian-fixes/pkg/xdrawchem/18c1ad2d-531b-4c4e-bd86-0db9cfb74ceb.