Users should be able to leverage warnings on videos

# Description

As it goes for Wikipedia, users with a high reputation should be able to leverage warnings on videos.

We currently have such a warning displayed for unlisted videos:


# Specs

- [ ] Statements are de-synchronized
- [ ] Not enough participation
- [ ] Possible astroturf
- [ ] Not enough sources
- [ ] Too much opinion
- [ ] Too much oriented for one side

---

:information_source: See https://en.wikipedia.org/wiki/Wikipedia:Template_messages/Disputes for a list of all Wikipedia's warnings.

---

Migration for API:

```elixir
defmodule DB.Repo.Migrations.CreateContentWarnings do
use Ecto.Migration

def up do
DB.Type.ContentWarningSeverity.create_type()

create table(:content_warnings) do
add(:video_id, references(:videos), null: false)

# Is it a success message (eg. "Good verifications here") or a severe warning (eg. "Astroturf")
add(:severity, :content_warning_severity, null: false)

# Type will be defined in frontend for easier modifications
add(:type, :string, null: false)

# A custom message that users can set to explain why this warning exist
add(:custom_message, :text, null: true, max: 4096)
end
end

def down do
drop(table(:content_warnings))
DB.Type.ContentWarningSeverity.drop_type()
end
end
```