Fixtures update

This update corresponds to 'fixes to Fixtures upload through API'. All of the [issues](http://manage.dimagi.com/default.asp?64973#371706) from the previous PR are fixed and tested.
- Fixed malformed document uploads giving 500 errors. Added a strict Fixture schema consistency check across all sheets
- Added the ability to add new Types, modify and delete using excel-file.
- Updated messages to be more helpful and relevent
- Linked upload/download instructions to a documentation page.

Few notes for future modifications
- In case of large fixture data, upload is successful, but takes lot of time (more than 30 minutes for a 18402 row fixture). So it's better, in case of large files, to add the upload task to a queue and notify the user once the upload is finished.
- Viewing large fixture data using webpage times out. So it's a good idea to not to display Fixtures with more than few 100 rows.
- Add a collapsible error-message holder.