Added navigation preview for the blog chnage form navigation tool.
Added tagit widget for blog categories.
Renamed field creation date to publication date.
Fixed issue where navigation preview not shown when adding node for the first time.
Added validation for pub dates on entry form. Also, as per requirements, publication date should be reset when entry gets published.
Blog author field is now an actual user. Added select2 field for author on entry forms.
replaced author var name with user. mistake when moving code around.
Categories now available in entry change form.
Added custom settings that is used to fetch allowed sites for users for specific models.
Users should not be allowed to add/change/delete entries from blogs where they are not allowed.
Merge remote-tracking branch 'origin/master' into LUN-1483

Conflicts:
	cms_blogger/models.py
	cms_blogger/settings.py