Move task-query to its own module.

Closes #453.
Merge pull request #468 from 4teamwork/deif-task-query-rewrite

Move task-query to its own module.
Add reworked `deadline_label` implementation to Task class.
Reworked sqltablelisting textfilter functionality.

Define searchable_columns in the BaseTableSource instead
of guessing the columns on the configured helper.
Dropped `overdue_date_helper` and replaced with lambda function.
Added taskquery option `by_brain`.
Reworked _get_task_css_class helper, it uses now task objects `get_css_class` method.
Fixed css_class getter in the task byline.
Reworked `Task.get_link` use css_class getter of task, instead of passing it by.
Merge pull request #474 from 4teamwork/pg_css_classs

Rework Task css class helper
Remove opengever-view.
Remove `get_client_id` from advanced search form.
Fix copy-paste error.
Cleanup imports, rename test.
Add depth argument to get_subdossiers to allow recursive traversing.

This fixes a potential bug with filing number generation for nested sub-dossiers.
Drop archiver mock test in favour of a functional test case.
Remove `get_client_id` from opengever.dossier.