Add thumbnailing tool for cropping one slice and saving the URL to its view
Add adjustable composite tile layer

This commit adds a an "Adjustable composite" link below all the stacks of a
project. This link allows to create on-the-fly tiles based ot the intensity
settings in the GUI. There, sliders allow to adjust the intensity of each
stack in the project. Dapending on the stack(s) this could involve quite
some memory and CPU requirements. The server configuration might be
tweaked to cope with that.
Thumbnailing tool: allow to export multiple stacks and default to 'all'
Thumbnailing tool: add marker creation functionality
Cropping tool: use microns as output unit
Add a link to the internal Redmine instance to the front page
Add notes and links to an english and a german imprint
Adjustable composite: add job class and separate processing function
Add gevent WSGI server run script
Support thresholds for adjustable composite display
Thumbnail tool: add testis folder
Adjustable composite: allow maximum intensity of 1000%
Import script: allow recursion into sub directories
Mass import script: optionally link to all users
Let mass import script deal with new Django permisson management
WIP: Let mass import script deal with tags
Add adjustable composite link to data view types
Tile processing: add expire header information
Adapt to the changed project/stack data structure indexing
Add adjustable composite entry to Projects>Current menu