[INTPROD-9204] Allow Match By Bot User ID

[colinsl]

This pull request includes changes to the omnibot service and its documentation. The changes mainly revolve around the addition of a user_id property in the Bot class, the modification of the _parse_payload method in the Message class to include a check for the bot's user_id, and the addition of important notes in the Adding normal slack apps documentation.

Documentation Updates:

• docs/root/adding_new_slack_apps.rst: Added notes about the potential for the bot to be called twice if app_mention is added, the need for bot.name to match the app user_handle, and the need to specify match_mention: true if user_handle and bot.name do not match.

Codebase Updates:

• omnibot/services/slack/bot.py: Imported get_auth from omnibot.services.slack and added a user_id property to the Bot class that fetches the user_id from bot_data or calls get_auth if user_id is not present. [1] [2]
• omnibot/services/slack/message.py: Modified the _parse_payload method to check if the bot's user_id is mentioned in the message.
• tests/unit/omnibot/services/slack/message_test.py: Updated the test setup to include a user_id in the bot's data.

[colinsl]

PTAL @ryand0 @david-lyft @toverturf-lyft