Migration and Model added. Refs #371

This commit adds the migration, updates the model and also the app presenter and tasks to participate.
Update and Create methods updated. Refs #371
Unit Tests. Refs #371
Unit Tests. Closes #371
Updated docs. Refs #371
Merge pull request #420 from PyBossa/issue-371-force-signin

Issue 371 force signin
Update README to be more clear about DB migrations
Do not include hidden apps.
First public profile commit. Refs #369
Refactor the grid apps method. Refs #369
Include also users without contribs. Refs #369
Integration of public user profiles with leaderboard. Refs #369
Link public profile from app.stats. Closes #369
Merge pull request #422 from PyBossa/issue-369-public-user-profile

Issue 369 public user profile
Link also in front page public profiles. Refs #369
Merge branch 'master' into culttag-redux