Connectivity widget: don't show synapse count when it is zero

Instead of showing zero for no synapses to a partner, the table cell is
now displayed empty. This reduces the visual noise when looking on
larger tables. This fixes #664.
Split/merge dialog: destroy 3D viewer when closed without "cancel" button

The 3D viewer wasn't destroyed when one closed the dialog with the help of the
little X in the upper right corner. This broke the 3D viewer and made it not
appear on subsequent dialogs. This commit fixes this by using a proper close
method like suggested by the jQuery UI API documentation. In there the whole
dialog is destroyed, including the 3D viewer. This is related to issue #605.
Merge remote-tracking branch 'upstream' into jankfree
utils.js: new function to parse color object from Raphael.colorwheel
Group Graph: prettier graphics for edges, with editable properties from the dialog.
Updated cytoscape.js to 2.2.8
Group graph: new layout modes concentric:

 1. concentric (degree)
 2. concentric (out degree)
 3. concentric (in degree)
Group graph: increase node spacing in concentric layout.
Group graph: fix error setting color of group.
Fix consitency migration for review table

This migration was committed not working. A single model object doesn't
have the "update" method. This method can only be used on query sets.
This commit fixes this by explicitly updating the models field and
calling save() on it.
Group graph: fix error preventing synapse clustering.
Merge branch 'master' of github.com:acardona/CATMAID
Arbor: added function to compute Strahler analysis
webglapp: new shading by the Strahler index of each branch.
WIP
Neuron navigator: support annotation removal from multiple neurons

This commit adds a new button to the annotation page of of the neuron
navigator. Sitting on top of the table of neurons that are annotated
with the current annotation, this button allows one to remove the
annotation from the all selected neurons. A user is required to have
permission to remove the annotation from *all* neurons, before any
annotation is actually removed.

If there are co-annotations selected, a button is shown for every
annotation contributing to the current filter set.

This is related to issue #654.
Connector table: add last modified column

This fixed #685.
Fix skeleton graph calculation by using new review system

If the risk calculation was selected, the skeleton graph calculation was
still expecting the old review system. This commit fixes this problem
and is part if issue #580. Thanks to Stephan Gerhard for pointing this
out.
Graph widget: fix node coloring for risk calculation

The back-end provides a single skeleton ID for each node, but the fron-end was
expecting a list. This commit makes the front-end use the single skeleton ID
provided by the back-end instead of the first list element.
Annotation removal: don't require permissions for all links in request

When annotations are un-linked from a list of entities, one was required
to have permissions to remove the annotation from every entity, before any
actual removal.  This commit changes this behavior: Now all links are
removed for which a user has permissions, the others remain untouched.

This was discussed in issue #654.