Provide administration page

[thombergs]

The "Administration" Link in the navigation menu currently creates an error. It should display a page with the following information:

Import Jobs

• currently running jobs (see org.wickedsource.gitanizer.mirror.importing.SubgitImportService.COUNTER_ACTIVE_TASKS)
• currently queued jobs (see org.wickedsource.gitanizer.mirror.importing.SubgitImportService.COUNTER_QUEUED_TASKS)
• max parallel tasks (see org.wickedsource.gitanizer.mirror.importing.SubgitImportService.MAX_PARALLEL_TASKS

Delete Jobs

• currently running deletion tasks (see org.wickedsource.gitanizer.mirror.delete.DeleteMirrorController#COUNTER_ACTIVE_TASKS)
• currently queued deletion tasks (see org.wickedsource.gitanizer.mirror.delete.DeleteMirrorController#COUNTER_QUEUED_TASKS)
• max parallel deletion tasks (see org.wickedsource.gitanizer.mirror.delete.DeleteMirrorController#GAUGE_MAX_PARALLEL_TASKS)

Working Directory
It would be nice to be able to see all subdirectories of the configured working directory and the disk space they occupy. Each directory should be marked if it is used by a mirror. If not, provide a "delete" button so it can be deleted manually. This is helpful, since deleting a mirror sometimes does not delete the working directory correctly (for example, if it is blocked).