Classifications: add support for root retrieval of project lists
Add admin tool "Tag Based Classification Graph Linker"

The tool introduced by this commit adds a new custom admin view with
which one can link classification graphs to projects in an automated
fasion. This is done based on the attached tags of a project. The tool
finds tag sets and offers to unify the linked classification graphs
or projects within those tag sets.
Add optional importer step to link suggested classifications

This commit introduces a new optional option called "Check
classification links". When checked a new importer step is added after
the project selection. This step suggests existing classification graphs
to be linked with the new project(s). The suggested graphs are project
independent and have the the same tag set (or a super set of it) as the
new project(s). The user is then free to mark those classification
graphs for automatic linking after the final confirmation step.
Fix small typo in documentation
Merge branch 'classification-admin-tools'