Resolve "Implement extra work declaration workflow"

Closes #27 (closed)

This MR adds the feature of extra work. Managers of courses can add extra work and then everyone can use submit declarations for that extra work. The managers then approve or reject the declaration.

Page changes:

• redesign of user and manager view of job offers to now include extra work
  • jobs and work are now grouped per course edition
• new page for individual extra work
• new tab on the applications page to include the declarations of a user

Screenshots:

Redesigned Job offers page

New Submit Declaration overlay

New empty job offers page

Redesigned Managers page

Redesigned Create Job Offer / Extra Work overlay

New Extra Work details page

Redesigned Applications page

New Declarations page