Several improvements of functionality and UI usability

Following is a list of major changes:
- added support for modified month ocurrences (e.g. "on 10th day before month end", "on second Tuesday of month"),
- removed "show" recurrence view. In my opinion it was duplicating what has already been shown in "issue" view and added unnecessary one more click to get to "edit" view.
- modified task generation process (invoked through cron) to create next recurrence as soon as previous recurrence day is reached. This way next task recurrence is known in advance (not 1 day before its due date as before). It is important to allow planning issue execution efficiently in real life.
- partly changed UI design to better match native Redmine look and allow more efficient usage of plugin,
- other minor changes
Need to use backwards-compatible format of :if => instead of if: -- had previous issues finding all occurrences of the newer syntax because many are on older versions of Rails.