Test failed locally in `TestPublicChannelMention`. This test seems to
specify that the same message may arrive twice with the same ID, so
rather than overthinking this one I propose we just use `FirstOrDefault`.
```csharp
TearDown : System.AggregateException : One or more errors occurred.
(Sequence contains more than one matching element)
----> System.InvalidOperationException : Sequence contains more than
one matching element
--TearDown
at System.Threading.Tasks.Task.ThrowIfExceptional(Boolean
includeTaskCanceledExceptions)
at System.Threading.Tasks.Task.Wait(Int32 millisecondsTimeout,
CancellationToken cancellationToken)
at osu.Framework.Extensions.TaskExtensions.WaitSafely(Task task)
at osu.Framework.Testing.TestScene.checkForErrors()
--InvalidOperationException
at System.Linq.ThrowHelper.ThrowMoreThanOneMatchException()
at System.Linq.Enumerable.TryGetSingle[TSource](IEnumerable`1 source,
Func`2 predicate, Boolean& found)
at System.Linq.Enumerable.SingleOrDefault[TSource](IEnumerable`1
source, Func`2 predicate)
at
osu.Game.Overlays.Chat.DrawableChannel.<processMessageHighlighting>b__14_0()
in
/Users/dean/Projects/osu/osu.Game/Overlays/Chat/DrawableChannel.cs:line
102
at osu.Framework.Threading.ScheduledDelegate.RunTaskInternal()
```
Adds new component `ChannelControlItem` and it's child components to be
used as the clickable control in the new chat sidebar for joined
channels.
Has public properties `HasUnread` and `MentionCount` to control the
display of the channel having unread messages or mentions of the user.
Channel select/join requests are exposed via `OnRequestSelect` and
`OnRequestLeave` events respectively which should be handled by a parent
component.
Requires a cached `Bindable<Channel>` instance to be managed by a parent
component.
Requires a cached `OveralayColourScheme` instance to be provided by a
parent component.
