Feature/sections primaryfix

This pull request contains changes for : 
1) A new  "is_primary_section" to check for primary sections. The primary sections  check was in various places and they have been modified to use this utility. Some flags are being set based on this check and passed into the templates to hide/show links.
2) The greying out of delete after editing a section has been fixed by adding back the slugify filter that had been deleted(Issue 2 in google docs)
3)Modified the confirmation text on add user (Issue 3 in google docs)

Note that a lot of unit test cases are broken due to signature changes in many methods to pass this new flag. The fix for the unit tests and new ones will be submitted as another request
Just an observation. People manually creating sis_section_ids for the purpose of doing an import will need to know not to name them with some number unless they want them to be primaries.