webgallery prototype from github/will-moore/webgallery

This prototype app has been started as a standalone web app repo.
Following discussion with UX group, we want to allow more widespread testing
and review of this code.
In future we will likely want to ship /webgallery as part of the main OMERO repo.
Adding webgallery to INSTALLED_APPS
Adding 'Gallery' link to TOP_LINKS in omeroweb/settings.py
Trivial typos not affecting behavior.
Clean up some Python use of deprecated raise "string"
remove unnecessary Python imports of exceptions
Fix zip creation.
rebase of jburel's d2b7c42
Fix missing negation in ExportActivity.

This wouldn't cause any problems as the higher-layer logic
already appends the .ome.tiff extension to the file name.
Merge pull request #1102 from mtbc/cli-import-error

fix uninitialized raw file store error on CLI import
merge dev_4_4 -D org -I include -E exclude: PR 988 (webgallery prototype from github/will-moore/webgallery)
merge dev_4_4 -D org -I include -E exclude: PR 1073 (Various code cleanup, mostly Python exceptions)
merge dev_4_4 -D org -I include -E exclude: PR 1089 (Fix zip creation.)
merge dev_4_4 -D org -I include -E exclude: PR 1091 (Fix missing negation in ExportActivity.)
merge dev_4_4 -D org -I include -E exclude

Repository: openmicroscopy/openmicroscopy
Already up-to-date.
Merged PRs:
  # PR 988 will-moore 'webgallery prototype from github/will-moore/webgallery'
  # PR 1073 mtbc 'Various code cleanup, mostly Python exceptions'
  # PR 1089 mtbc 'Fix zip creation.'
  # PR 1091 bpindelski 'Fix missing negation in ExportActivity.'

Repository: openmicroscopy/bioformats
Already up-to-date.
Merged PRs:
  # PR 486 rleigh-dundee 'ZeissZVIReader: Fix detection of label string offset'
  # PR 487 sbesson 'Fix licensing note in the SCIFIO legend'

Repository: ome/scripts
Already up-to-date.

Repository: openmicroscopy/ome-documentation
Already up-to-date.

Generated by OMERO-merge-stable#228 (http://hudson.openmicroscopy.org.uk/job/OMERO-merge-stable/228/)