Fix some issues reported by lintian

Fix some issues reported by lintian

• debian/copyright: use spaces rather than tabs to start continuation lines. (tab-in-license-text)
• Re-export upstream signing key without extra signatures. (public-upstream-key-not-minimal)
• Set upstream metadata fields: Bug-Database, Bug-Submit. (upstream-metadata-missing-bug-tracking)

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/07/a487d01fa22419f9648df8ea3399ec49ca1af2.debug

Files in first set of .debs but not in second

-rw-r--r--  root/root   /usr/lib/debug/.build-id/04/d0f80540620116425f719382ffca68ecff4dd8.debug

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

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

• Build-Ids: 04d0f80540620116425f719382ffca68ecff4dd8 07a487d01fa22419f9648df8ea3399ec49ca1af2

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Build and test logs for this branch can be found at https://janitor.debian.net/run/8605fd16-7d50-48c4-a78a-4c6f00a99d00.