Fix some issues reported by lintian

Fix some issues reported by lintian

• Trim trailing whitespace. (trailing-whitespace)
• Use secure URI in Homepage field. (homepage-field-uses-insecure-uri)
• Set field Upstream-Contact in debian/copyright.
• Set upstream metadata fields: Repository.
• Remove obsolete field Contact from debian/upstream/metadata (already present in machine-readable debian/copyright).
• Fix field name typo in debian/copyright (Comments ⇒ Comment). (field-name-typo-in-dep5-copyright)
• Update standards version to 4.6.2, 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 (out-of-date-standards-version)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/e0/192b7f75df923b66bd6c827c1ae5c8cd334310.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/79/9f65471453d68b5ee6f5d9d216ca94f3745f1e.debug

Control files of package avogadro: lines which differ (wdiff format)

• Homepage: http​://avogadro.cc/ https​://avogadro.cc/

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

• Build-Ids: 799f65471453d68b5ee6f5d9d216ca94f3745f1e e0192b7f75df923b66bd6c827c1ae5c8cd334310

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Build and test logs for this branch can be found at https://janitor.debian.net/run/596e327d-08a8-4ed8-8a67-16aeb6816ded.