Update app thumbnail
Use new avatar uploader forms. Closes #683

Upload for api will be addressed in #633
Download avatars from external URLs and serve them
Upload avatars from users and apps to PyBossa
Refactor
Remove comments
Use the new uploader forms
Patch uploader
Fix tests for new uploader forms
Fixes to pass tests
Update theme to use avatars
Merge branch 'master' into issue-683-app-avatar
Merge branch 'master' into issue-683-app-avatar
Refactor to use tempfiles
Patch for uploading files
Make more specific the upload method
Merge pull request #768 from PyBossa/issue-683-app-avatar

Issue 683 app avatar