Clear the session on setup start

The migration stores some values in the session, which causes a failed migration
to start from the point where it failed after a cleaned database. This can only
be fixed by clearing the session.
Rename projects with the same name under the same parent in migrations

We enforce unique project names under the same parent project since a few
versions via a database constraint.
fix the deletion handling and structure
stop migrating todos
stop migrating nodes
stop migrating filemanager entries
stop migrating contacts
stop migrating helpdesk entries
Stop migrating calendars

We just keep the module because we need some of the code, we don't really
provide a calendar any more.
Merge remote-tracking branch 'mayflower/next' into migration-169

* mayflower/next: (66 commits)
  fix _projectsParamToArray helper to use null as the default
  make getMonthRecords static
  Fix copy-paste error in Default_IndexController::_getDateStringParam
  Adjusted calling code after method renaming
  prefer-source in composer in .travis.yml so that it uses git
  add missing index.json.phtml
  remove getGlobalModules call in frontend
  fix startup sequence of the javascript
  change default value to null in timecard model project filter
  Fix an error in Project/Sql/Db.json
  commend copied blocks in projectchooser
  factor out projectChooser for statistics to enhance readability
  rename _onProjectChange function to _updateMonthWidget
  remove unnecessary dom emptying from monthgraph.
  add missing destroy checks in monthgraph and table
  make the monthgraph/monthtable load depend on the projectchooser change event.
  prevent change event emitting before the projectChooser data has been loaded. otherwise we have multiple change events that trigger reloads upstream
  don't use getModelObject anymore
  use the css class "button" instead of "submitButton" for our buttons
  use single quotation marks consistently
  ...

Conflicts:
	phprojekt/application/Project/Sql/Db.json
Merge pull request #364 from voidus/migration-169

Fix the migration for our current state