Replace findBySlack logic by findBySlackUserID logic

[ivshapovalov]

Store slack user ID in DB
Slack user ID is text. For example, UABC123.

Implement synchronizing with CRM new field Slack user id.
Rename all related with SlackName entities and variables on using SlackUserID for searching user.
Add slack user ID in UserDTO.
In CRM fields have names c_slack_id for slack user id and c_slack_display for slack user display name.