Add Slack Username registration feature

[otsuka]

• You can register the Slack username in the user's account settings.
  • Administrator needs to create a user custom text field named "Slack Username".
  • If Slack's username is different for each project, register it like "project_identifier_1:slack_username_1, project_identifier_2:slack_username_2".
• If Redmine's user ID and Slack's username are different, "@redmine_user_id" in an issue comment will be converted to the Slack username registered by that user and posted to Slack.
  • This feature is useful when used with Redmine Mentions plugin (https://github.com/arkhitech/redmine_mentions).
• When the assignee of an issue is changed, the message with "@assignee_slack_username" mention is posted to Slack.
• A hash for the comment is added to the issue URL posted to Slack when an issue is updated.

[sciyoshi]

Wow @otsuka, this is great! Thank you for the PR, I will take a look shortly.

[McAronDev]

Thank you, @otsuka!! Also it will be cool to add mentions for watchers