OpenXML: Translate hyperlinks stored in rels files

[Chase Tingley]

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:

  <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.

[Chase Tingley]

A related issue here is that even if we expose these, they're still not related as subflows/associated to the places in the body where they are referenced from. This would require rearranging TUs across different document parts, which would give the current structure of the filter fits.

[Chase Tingley]

Fix Issue #533: OpenXML: Translate hyperlinks stored in rels files

Squashed merge commit of Bogdan's external_hyperlink3 branch
See https://bitbucket.org/okapiframework/okapi/pull-requests/84/

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