Abstract dismissable banner to new table
## Problem
We currently have **548 columns** in `application_settings` and we store a lot of data which does not belong there.
As suggested in https://gitlab.com/gitlab-org/gitlab/-/merge_requests/172151#note_2215440641 we should create a new abstraction for this.
## Solution
Let's create a new table to persist dismissable banner at the instance level.
```ruby
class ApplicationSettings
has_many :dissmissable_banners
end
module AppConfig
class DissmissableBanner
belongs_to :application_settings
end
end
```
cc @jivanvl
issue