OpenXML: Translate hyperlinks stored in rels files

Some hyperlinks in Office documents are stored as field codes (which, as of current M30 snapshot builds, work correctly), but many others are stored in the .rels as relationships with TargetMode="External". Samples of this can be found in the attached samples -- in the word document, look at word/_rels/document.xml.rels, and for pptx, look at ppt/slides/_rels/slide2.xml.rels.

In both cases, the relevant links look like this:

#!xml

  <Relationship Id="rId5" 
Type="http://schemas.openxmlformats.org/officeDocument/2006/relationships/hyperlink" 
Target="https://bitbucket.org/okapiframework/okapi/" TargetMode="External"/>

When we expose hyperlinks for translation, these should also be included. (Also, the translation of hyperlinks should probably be a general option.)

I'm not sure if we need to scan all rels files, although that is probably the easiest thing to do.