Return a more precise error message

If there is not exactly one stack-project link found, tell the user if
it was none or more then one.
Importer: enlarge tag input field
Ontology editor: remove superfluous option from some context menus

For the sake of simplicity, the option to add a new class or relation
should only be available in the context menu of class and relation
tree's root nodes.
Bug in connector table API when no displaystart or length is specified.
Add project stack inline to stack admin page

This allows to view and edit the project-stack relation out of a stack's
admin page. All links referring to the current stack are displayed in a
table below the fields of a stack.
Add reference lines to default settings management command
Importer: make default tile width and height adjustable
Allow ontology relations to be renamed
Ontology editor: add relation renaming support
Allow ontology classes to be renamed
Ontology editor: add class renaming support
Merge branch 'rename-option-for-ontology-editor'
Show more reasonable ID in a CICI link's text representation

With respect to a classification root node, class_instance_b of a
ClassInstanceClassInstance (CICI) link is the actual root class instance
of a classification graph. Its class_instance_a would refer to the
linked project. Since there can be many CICI links to the actual root,
but from different projects, it is makes sense to not show the link's
ID, but the root's.
Importer: change style to fit more into admin layout

This commit also changes the user and group permission control elements.
Now the FilterSelectMultiple class is used there, which is in use on
other places in the Djange admin. This increases usability for multiple
selections on large lists.
Importer: allow manual selection of classification graphs to link

If a user chooses to link classification graphs on the first page, (s)he
now able to not only linke the suggested classification graphs, but also
any graph wanted.
Importer: increase width of classification graph control
Merge branch 'importer-update'
Update pgmagick requirement for Mac OS

Installation failed on Mac OS with pgmagick 0.5.5 (libboost library
could not be found because of an error in the install script). This was
fixed in 0.5.7
(https://bitbucket.org/hhatto/pgmagick/commits/977a2d1862553198d347464bb
7ca2ea7efcf6090)
Add ProjectStackInline to ProjectAdmin

ProjectAdmin now shows associated Stacks. Also renamed ProjectInline to
ProjectStackInline to better reflect the model name.
Merge remote branch 'felix/small-fixes'