Refactor: extract common code to link_to_month.
Add the project_id to the Calendar link to month.
Refactor: extract target link generation.
Refactor: extract ternary operators to temps.
Refactor: extract link to previous and next month into helpers.
Refactor: convert many of the custom Issue routes to REST resources.
Refactor: extract method in bulk_update.
remove BOM from zh-TW
Translation updates * de (#6198) * ja (#6189) * zh-TW (#6197)
Refactor: extract #bulk_update method from IssuesController#bulk_edit.
