Improve Document Preview and Document Selection [#319 responsible:none state:fixed]

Updated image preview so that the user can resize the file selection dialog
and get a better (and clearer) image preview. Also implemented file name filtering
when using the local file selection tab to select a file.
Improve performance of Server Image Preview [#320 responsible:none state:fixed]

Updated so that server image previews are cached on the client.  To prevent
images from becoming stale, the last modified date is pre-pended to the
file name. This will ensure the thumbnail is updated if the source image is
updated.
Merge branch 'master' of https://github.com/SOLA-FAO/clients