Move database events endpoint address to settings

What does this MR do and why?

Related to #408644 (closed).

This adds a database migration to add gitlab_database_collector_hostname column to ApplicationSetting. The setting specifies the hostname for the Snowplow collector that collects only database events, and an existing constant will be moved to that setting.

Since some code depends on the existing constant, the setting will still fall back to the constant if not set. After populating the gitlab_database_collector_hostname setting, the fallback will be removed (or perhaps be set to the gitlab_collector_hostname setting).

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