Allow elogEntries to be built incementally

Originally for SCATTER-30
Also allows images to be added within blocks of text rather than as a list at the end.

Previous functionality should be unaffected.
Fix for previous commit (946fd4501fb845bf87) to prevent new property being required in java.properties

If you need the incremental entries, add:
gda.elog.imageurl=http://rdb.pri.diamond.ac.uk/php/elog/cs_logonlyimageupload_ext_bl.php
to your java.properties
Correct property for getting file prefix

gda.data.filePrefix instead of gda.nexus.beamlinePrefix
Merge branch '8.38' of ssh://dasc-git.diamond.ac.uk/gda/gda-core into 8.38