Toggleable author selection for add page
* authors are now selected from existing authors by default (when adding)
* new file: common.js (moved missionSaveError & getMissionAuthor here)

ToDo:
* Allow selection of > 1 author
* The plus symbol's size & positioning needs to be tweaked a bit; maybe just use a normal "+" instead?
Merge pull request #29 from ferstaberinde/dev

Toggleable author selection for add page