Allow any combination of url & source in brandlogo

[ascholerChemeketa]

Implements todo in pretext-html.xsl

[ascholerChemeketa]

If no URL, tries to use $baseurl so that image behaves the same as the title it is next to. If that fails it resorts to linking back to the same page.

[rbeezer]

Very good. Thanks!

Some XSL nits:

• I put actions for xsl:when onto their own indented lines, as discussed at Drop-In.
• <otherwise/> works just as well for a no-action final eventuality.
• A != operator has bit me before. Maybe an empty node-set and an empty string both become false - I don't know. So I go

  not( [node-set] = '')

  made that edit here.

Likely website update before the day is out.