As seen by this kind of crash, having the `.ToMod` method throw can be
very problematic and also hidden (as it is used inside of models in
places where exceptions are not expected to occur).
Given there are tens of usages of this method, returning a placeholder
mod seems like a better idea than outright throwing.
```
An unhandled has occurred.
System.InvalidOperationException:
There is no mod in the ruleset (osu) matching the acronym AS.
at osu.Game.Online.API.APIMod.ToMod(Ruleset ruleset) in /Users/dean/Projects/osu/osu.Game/Online/API/APIMod.cs:line 54
at osu.Game.Scoring.ScoreInfo.<get_Mods>b__117_0(APIMod m) in /Users/dean/Projects/osu/osu.Game/Scoring/ScoreInfo.cs:line 193
at System.Linq.Enumerable.SelectArrayIterator`2.ToArray()
at osu.Game.Scoring.ScoreInfo.get_Mods() in /Users/dean/Projects/osu/osu.Game/Scoring/ScoreInfo.cs:line 193
at osu.Game.Screens.Select.Leaderboards.BeatmapLeaderboard.<>c.<subscribeToLocalScores>b__40_2(ScoreInfo s) in /Users/dean/Projects/osu/osu.Game/Screens/Select/Leaderboards/BeatmapLeaderboard.cs:line 199
at System.Linq.Enumerable.WhereEnumerableIterator`1.MoveNext()
at osu.Game.Database.RealmObjectExtensions.Detach[T](IEnumerable`1 items) in /Users/dean/Projects/osu/osu.Game/Database/RealmObjectExtensions.cs:line 180
at osu.Game.Screens.Select.Leaderboards.BeatmapLeaderboard.<>c__DisplayClass40_0.<subscribeToLocalScores>g__localScoresChanged|1(IRealmCollection`1 sender, ChangeSet changes, Exception exception) in /Users/dean/Projects/osu/osu.Game/Screens/Select/Leaderboards/BeatmapLeaderboard.cs:line
209
at Realms.RealmCollectionBase`1.Realms.INotifiable<Realms.NotifiableObjectHandleBase.CollectionChangeSet>.NotifyCallbacks(Nullable`1 changes, Nullable`1 exception)
at Realms.NotifiableObjectHandleBase.NotifyObjectChanged(IntPtr managedHandle, IntPtr changes, IntPtr exception)
```
Adds components `ChannelListing` and `ChannelListing` item with visual
test. Essentially a more simplified version of the existing
`ChannelSelectionOverlay` component.
Correctly implements `IFilterable` behaviour to filter child channel
items. Channel joined state is based on the underlying `Joined` bindable
of the `Channel` class.
Channel join/leave events are exposed via `OnRequestJoin` and
`OnRequestLeave` events which should be handled by parent component.
Requires a cached `OverlayColourScheme` instance to be provided by the
parent overlay component when added.
