When checking whether a node is root, don't rely on the parent node existing.

Happens frequently that one selects a node, scrolls away multiple sections,
and while that node is loaded (because it is selected) with the rest,
its parent won't be.
goprevbranch needs the event as argument.
Added pillow==1.7.8 to pip-frozen, to include PIL.
New functions _rootID(graph) and _edgeCountToRoot(graph)
in skeletonexport.py, to find the root node in a NetworkX.DiGraph
and to obtain a map of nodeID keys vs count of edges to root as value.
Skeleton treenode review system: create chunks continuous across branch nodes,
and always traversing from end node towards root.

The longest path will traverse from the furthest end node to the root node.
The second longest will traverse from an end node to a branch point that
has already been seen. And so on.

The desired effect: create less sequences to inspect which minimizes context
switching (changing of location in the image series), and also not creating
a long list of short sequences between branch points.
alt+v and alt+b go to the next or previous interesting treenode,
where interesting is a treenode with:
 * confidence less than 5
 * a treenode_classinstance.relation_id, like labeled_as
 * a treenode_connector.relation_d, like presynaptic_to or postsynaptic_to relation
Update httpdocs/webglapp.js

Disable broken textlabel visualization
Remove try/except block from cropping imports

The try/except block was meant to make things easier for people who
don't want to or don't need to use Celery. The idea was to treat
import errors of it silenly. However, since there were multiple
imports in this try/except block, a failed import of the first
import would cause the second (and third) not to be even tried. To
not further complicate things, I just removed the try/except block
and expect Celery to be configured. This shouldn't be a problem
since the default settings file already provide a minimal working
configuration with djcelery.
Merge branch 'feature/ontology-editor' into config-eaton-features
Add thumbnailing tool for cropping one slice and saving the URL to its view
Add adjustable composite tile layer

This commit adds a an "Adjustable composite" link below all the stacks of a
project. This link allows to create on-the-fly tiles based ot the intensity
settings in the GUI. There, sliders allow to adjust the intensity of each
stack in the project. Dapending on the stack(s) this could involve quite
some memory and CPU requirements. The server configuration might be
tweaked to cope with that.
Thumbnailing tool: allow to export multiple stacks and default to 'all'
Thumbnailing tool: add marker creation functionality
Cropping tool: use microns as output unit
Add a link to the internal Redmine instance to the front page
Add notes and links to an english and a german imprint
Adjustable composite: add job class and separate processing function
Add gevent WSGI server run script
Support thresholds for adjustable composite display
Thumbnail tool: add testis folder