Fixed CKEditor stuff:

- Added missing insertpre plugin and allow <pre> tag with class
  attribute. Added font style to base.css.
- Removed unused link to old ckeditor.css (now managed by
  django-ckeditor).
Fixed participant csv import for group id

- Allowed to add multiple groups in csv group id field (e.g. "3,4")
- Fixed bug that group ids > 9 can not be imported.
- Updated error message if group id does not exists.
Show supporters on motion slide if available.
Merge pull request #1329 from normanjaeckel/fixes-for-1.6.1

Fixes for 1.6.1
Omit leading zero in method item_no if no agenda prefix is given.
Fixed #1326: Assignment PDF fails when having a lot of posts.

Print always 7 signature lines in assingment pdf.
Added test.
Merge pull request #1340 from emanuelschuetze/fix-1326

Fixed #1326: Assignment PDF fails when having a lot of posts.
Fixed motion detail view template. Added block to enable extra content via plugins. Updated CHANGELOG.
Merge pull request #1338 from normanjaeckel/FixedMotionDetailViewTemplate

Fixed motion detail view template.
Merge pull request #1341 from frauenknecht/i1324

Fix #1324 - Agenda item has whitespaces before number.
Updated CHANGELOG.
Bugix in method get_title. Return item no for related objects.
Merge pull request #1350 from normanjaeckel/UpdateChangelog

Updated CHANGELOG.
Merge pull request #1351 from frauenknecht/i1327_2

Fix #1327 - Related agenda item has no (auto) agenda number
Updated English language files.
Returns a 404 response if a requested poll does not exist.

fixes #1355
Merge pull request #1358 from normanjaeckel/LanguageFile

Updated English language files.
Merge pull request #1356 from ostcar/poll404

Returns a 404 response if a requested poll does not exist.
Merge branch 'stable/1.6.x'

Conflicts:
	tests/assignment/test_views.py