Fix some errors
Fix Bug #828: Validate username
Update the sampledatahelper version and use file_from_directory
Fix #830: Remove token of endpoint memberships
Fix bug #726: Solve problem with with attachments content-type header
Fix bug #726: Solve problem with with attachments content-type header (again)
Fix bug #827: Improve invitation emails
Fix bug #783: Verify email existence on membership objects
Add access-control-max-age header to cors middleware for avoid options pre-flight.
Merge pull request #64 from taigaio/avoid-options-pre-flight

Add access-control-max-age header to cors middleware for avoid options pre-flight
Fix bug #752: Show only description_diff in emails
Fix bug #831: Assure us order change is registered in the history independently
Fix bug #868: Delete empty description_diff
Ignore .cache dir
Remove unused import.
Disable not partial updates on attachment resources.

PUT request does not supports multipart request, but attached_file
is required. Because of this condition, put requests for attachments
resources are not possible, for it, them are disabled explicitly.
Check if project matches the content_object.project.
Remove deprecated attributes on attachments resources.
Remove unsed import on attachments permissions module.
Set attached_file field on attachments serializer required.