'Fix import errors during production docker build' into release/1.14

Ghost User
2018-03-16T15:12:48+01:00

Fix import errors during production docker build

Closes #611

See merge request thalia/concrexit!762

(cherry picked from commit 126f7201cad74eb91cb90d8b5ff0e7bcff8c6043)

c00b7945 Remove multiprocessing Sphinx builds
bc654df1 Use strin...