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.
Graph widget: invalidate Cytoscape's canvas dimensions on resize

Telling Cytoscape explicitly about the resize is required, because it
doesn't monitor the dimensions of the viewport (which makes sense,
because it can be expensive). This fixes an issue which, since the last
Cytoscape update, caused the canvas to stay at the same size as before,
when the widget size was changed.
Graph widget: reduce the redraw timeout after resize to 100ms

This will cause a redraw to happen faster after a resize, but keeps
resizing still smooth. The former value of 400ms felt a bit to large
if one clicked right away into the graph after a resize. If Cytoscape
wasn't informed about the dimension change yet, the mouse positions were
wrong. This doesn't happen if the update is scheduled faster.
Statistics widget: improve performance by avoiding a join

If one doesn't query for the user names for each count directly, but uses only
the user IDs, one can save quite a bit on larger tables. Querying for the user
name directly implies a JOIN. Since we have the ID to name mapping available
anyway, there is no need to query for the names directly.
Graph widget: update resize timeout comment
Statistics widget: improve performance by better date handling

Up to now the dates queried and used by the statistics widget used to
Postgres' to_char() function to bring the date into a YYYYMMDD format.
Ignoring the time of the day is required to have meaningful statistics
per day. However, using Postgres' date_trunc() function is much faster
for the actual query. The transformation to the YYYYMMDD format is then
done in Python. This introduced a speed-up of about a factor of ten in
my test cases.
Selection table: hide color wheel properly for new rows

The DIV containing the color wheel should be hidden by default---not
only the color wheel itself. This wasn't the case before and it caused a
line break right in front of the info button. With this commit the DIV
is hidden when added and no more line break is shown.

Thanks to Javier Valdes Aleman for pointing me to this little annoyance.
Merge branch 'sholl'
Merge branch 'master' of github.com:acardona/CATMAID
Updated morphology_plot.png to "MP"
Morphology analysis: in the case of using the highest signal flow for center,
there are going to be many nodes with the same value. Pick the one
with the highest betweenness centrality.
Morphology Plot: default center must be the first branch node.

In vertebrates, this is the soma.
In fly, it's the node closest to the axon hillock,
  and the least variable in spatial position.
Morphology plot: fix error in computing center with flow centrality.

Javascript's divisions are floating-point!
Morphology plot: export CSV
utils.js: generic function saveDivSVG
Group Graph now uses saveDivSVG.