Make location relation independent of concept relation

This commit adds a database migration to make location not inherit
concept anymore. It relates to issue #715. Doing this allows us to have
different data types for e.g. the ID of concept based relations and
location based relations. Of course, location.id also gets a new default
sequence, starting from the next free ID in location. The documentation
has been adjusted.
Update data types of many relations

As discussed in issue #715, there were quite some data types that are overkill
for what is required. This commit changes many data types of many different
database relations.
Update database schema documentation
Reorder columns of location based tables

This commit re-creates the table layout of the location, treenode, connector and
region_of_interest tables. The tables are optimized for data alignment to waste
as little space as possible. As discussed in issue #723 this saves 66 Bye per
row.

After this migration is applied (which could take some minutes, depending on the
size of the location based tables), you should run VACUUM FULL from  with
Postgres.
Let back-end read the changed database location columns correctly
Replace use of Double3D for node locations in backend
Fix more location data type references in backend
Fix more location references
Test data: update location representation
Test data: replace all unset editors with user of same objects

In a recent migration the editor database field got a NOT NULL
constraint.  Therefore, the test data had to change as well. FWIW, this
is the regex I used to replace all occurrences in vim:

%s/\("editor": *\)null\(, *\n *"user": *\)\(\d*\)[,]*/\1\3\2\3\4
3D viewer: allow transparency in neuron color

Add a transparency slider to the color wheel (see aschampion/colorwheel)
tied to basic opacity support in the WebGL viewer for manual
single-skeleton and group coloring. Closes acardona/CATMAID#592.
3D viewer: select mesh color with colorwheel

Replace the text input for mesh color with the interactive colorwheel.
Correctly parse the default mesh color to be white, as previously
configured. Correctly create the mesh material for transparency and use
the previously configured opacity of 0.2 by default. Support setting
opacity via the color wheel alpha slider.

Alternative solution to acardona/CATMAID#615.
Merge pull request #723 from acardona/db-data-type-changes

Change data type of columns in many relations
Fix retrieval of review information for a skeleton

This was introduced in 94723de by replacing values_list() with values(),
closes #755.
NeuroML export: fix indices to get skeleton ID from query

These indices changed when location was not a single field anymore, but was
replaced by three explicit columns.

Closes #760.
Fix off index in retrieval of open leaf nodes

The index shift was introduced through the layout change of location based
tables.

Closes #758.
Recognize empty location values from log table

Closes acardona/CATMAID#759.
Tests: fix project and stack insertion tests
Tests: fix authentication test
Tests: assign new test user required permissions

The test user must be able to browse and annotate in this test case.