Mass-update other SlackIntegration records with bot token when Slack re-installed

About

This issue came out of this discussion &7729 (comment 936450511).

Due to the current modelling of the Slack integration, when they re-install their Slack app (#360681 (closed)) they will need to take that action for each project that has the "GitLab Slack Application" integration.

Instead, we could mass-update any other SlackIntegration records that share a team_id and insert the new bot_access_token into these records on their behalf. See &7729 (comment 936450511) for untested concept code (note, that the integration we would scope for would be Integrations::GitlabSlackApplication and not Integrations::Slack in the comment).