Connectivity: make OR default, and rename to "All partners".
AND becomes "Common partners".

Makes sense to have OR as default given the "append" button.
Fix error in Selection Table.
New library file arbor.js to create and manipulate arbors.
An Arbor is defined as a graph:
 * with only directional edges
 * only one root node (one node without parent)
 * no loops.

This data structure allows for a number of high-performance versions
of otherwise slow generic graph algorithms.
Add documentation for the proxy_redirect command of Nginx

This is command is needed in an Nginx configuration if CATMAID is
living in a subdirectory (i.e. http://example.tld/catmaid/).
DB init script: re-add PLpgSQL language init and marker

Both, the language initialization and the end marker, has been removed
in b1d77aa3, supposedly by accident. This commit re-adds it again.
Readd synchronize object tree again. Closes #563
webglapp: better handling of mouse event functions and their removal.
webglapp: don't show the active node if not visible.
Would get shown when updating its position.

While at it, fix the logic in the overlay: only invoke the method when
a treenode is activated.
object tree: properly expand to selected skeleton upon initialization
and remove some friction by caching a reference to some DOM elements.
Merge branch 'master' of github.com:acardona/CATMAID
Removed synchronization for Treenode and Connector Table widgets.

Was adding friction to tracing for very little gain. The 'Refresh' buttons
are there for this purpose. Plus, refreshing a table with several thousand
treenodes every time a new node was added or removed was never a good idea.
overlay: fix error invoking the opening of the path in ObjectTree.
object tree: ensure the widget is open prior to refreshing it.
overlay: re-enable highlighting skeletons in Selection Table, Graph, etc.
but only if the activated node belongs to a different skeleton.

Avoids potentially expensive operations when adding nodes to the same skeleton.
Graph: report on the number of disabled nodes in the "Show hidden" button text.
Graph: reimplemented server back end to speed up basic case

The basic case does not split skeletons in any way,
therefore there is no need to retrieve all treenodes of every skeleton.

Additionally, the synapse clustering is applied only when appropriate.
Overall the new approach attempts to minimize operations at the server side
as much as possible.
Graph: add coloring modes

* by source (default)
* by review status (like in the connectivity widget)
* by input/output ratio (red: only outputs, green: only inputs)
* by betweenness centrality (using a jsnx.DiGraph)

Also fixed a number of incorrect uses of node.id() when node.data('skeleton_id')
was meant.
Graph: fix error in coloring by I/O
Graph: with the new scheme, circles must provide the neuron names.
Graph: ensure skeleton name is updated only if present in the provided model.