Add filtering for sending messages by issue tracker and status

[Anuovec]

Added ability to send message only for specified trackers or statuses.
E.g. I want to see, if there is a reported bug (tracker: bug, status: new) and if someone took it (status: in progress), but i do not need to know, that it done.

Similar to #99, but I changed the text field to multi-select, as you pointed out and added trackers.
Also, this resolves #96

[alexandermeindl]

Hi @Anuovec,

I'll accept your PR. Can you fix Rails version for migration to 6.1. Use use ActiveRecord::Migration[4.2] in your PR.

[Anuovec]

Yup, no problem. Done.

[Anuovec]

I found a bug.

In send_messenger_update, there was:
return if !filter_issue_trackers.empty? && filter_issue_trackers.exclude?(status.id)
instead of
return if !filter_issue_trackers.empty? && filter_issue_trackers.exclude?(tracker.id).

Fixed.