Allow project bulk edit of time entries
This issue follows #588#note-28 and adds the project field to bulk edit mode for time entries.
- Activity field is updated on project change (actually, the entire form is replaced on project change)
- Issue autocomplete is available only for bulk editing of time entries from the same project
- Added the "Clear" checkbox to the issue field.
- Moved the method "bulk_edit_error_messages" from issues_helper to application_helper because now it is used by both entities (issue and time entry).