use annotated fellow queryset to defeat N+1 problem

fixes #346

reword assignment stage update email
disable automatic withdrawal after assignment deadline

fix refereeing reminders not being sent

fix appraisals count not showing
hide appraisal qualification count
prefer accepting offers of least overloaded fellow
skip deprecated test

fixes #26

fixes #340

test conditional assignment offer factories

fixes #339

fixes #337

fixes #338

complements #336

fixes #336

add assignment deadline extension button for edadmin/authors

fixes #335

fix crash on profile merge