Migrate MR-related Project settings off projects table
We have Project-level settings scattered across 2 tables - `projects` and `project_settings`. `projects` is a really wide table with many, many rows; maybe we should think about migrating and consolidating them into either `project_settings` or create a series of tables for each major section. This would benefit `projects` itself, and give us an excuse and opportunity to take a pass at documenting and rationalizing the values of these columns; boolean in nature, we have inconsistencies in whether, say, a value of `true` is enabling or disabling of that setting.. it has tripped us up in the past. This work, if we were to take it on, should be done in consult with ~"devops::manage" and ~"devops::plan", both of whom have project-level settings of their own. Initial discussion: https://gitlab.com/gitlab-org/gitlab/-/issues/344632#note_958851466
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