convert OUString** in HtmlExport to std::vector<OUString>

There is no point in storing pointers to a ref-counted value type like
OUString.
And while we're there, convert arrays to std::vector

Change-Id: I96ceeb13dd3c4fb4f2ebdd017edc92d01dcb47cb
Reviewed-on: https://gerrit.libreoffice.org/7044
Reviewed-by: Michael Stahl <mstahl@redhat.com>
Tested-by: Michael Stahl <mstahl@redhat.com>