Add a field candidate_license to the Candidates table
Add a candidate_submitter in case the author of the artwork is not the submitter
Increate configuration options

- Add a list of extensions allowed for the files uploaded
- Add a list of MIME types allowed for the files uploaded
- Add a minimal width required for the image uploaded
- Add a minimal height required for the image uploaded
Add a form to submit candidates
CSS improvements

- Add a warnings class
- Add the css for the contribute tab
- Add a small class
Add the index page for the contribution
Add the template for submitting a new candidate
Fix the action of the form when editing an election
Update the add_candidate method to accept the new fields added to the database
Fix editing elections
Add a method to retrieve a canidate by name and election
When adding a candidate, raise an exception if the name is taken
Add method to retrieve the election existing and opend for submission
Small layout change in the list of elections opened for contribution
Add the contribution tab and logic

One can now log in onto nuancier, submit a wallpaper.
This submission is checked for the file extension, the mime type and its size.
If it all goes well it's added to the database and saved
on the disk.