always create default users

For old notebooks, there was no default user
and so certain functionality such as documentation
would not work with recent sage.
This patch makes sure that the default users are
always created if they are not present.
Update version in setup.py to 0.10.8.3
Update the changelog with a brief summary of changes
Merge pull request #193 from ppurka/issue-default-users

always create default users
Themes infrastructure
themes rendering and Test theme
themes running except worksheets
Themes on worksheets - Help added
Themes added and Test theme removed
Added themes data files to setup.py
Support for Samuel Ainsworth's new UI themes
Samuel Ainsworth's new UI added as a theme
Templates updates in sync with master for Non-NewUI themes
rebase conflict solved
Prepare themes for issue #210
preparing themes for different error messages when adding users
problem with translations in themes
more problems with translations in themes
more on upload translation in themes
sage newUI converted is a theme