Admin users database sort and paginate

In the admin, the users database page now allows sort clicking on the table header.
Also supports pagination.
Admin: adds a tab for data loaders

In the search for users and in the all users in the admin, a new tab for Data Loaders is added with sort and pagination.
Refactor code

Use a mixin for fixing the date creation column in users search and dataloaders search results.
Merge pull request #86 from camilotorresf/fixes

Users database page with pagination and sorting