Allow pay range selection by month or by year in board/jobtype settings

[jace]

For short term jobs like freelance and internship opportunities, pay range may be better defined on a monthly rather than annual basis.

Hasjob's internal storage is always annual, but the UI could switch to monthly where relevant. This could be specified in two places:

1. On the job type. When posting a job, after selecting the job type the UI changes automatically to reflect monthly range.
2. On the board. The drop-down filters then show monthly range instead of annual range.