Tracing: add shortcut to annotate active neuron

Make F3 bring up the annotation modal dialog for the active skeleton.

Closes acardona/CATMAID#752.
Tests: fix imports in common api tests
Test data: add missing user objects

This commit adds an anonymous user and a test user to the test data
base. These are required, because other objects refer to these objects
with a foreign key constraint.
Test data: add another missing test user

I forgot to add this in the last commit.
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