Reactions: Add more button (#1314)

* Add expand/collapse UI for reactions

- Adds a CollapsibleFlowLayout for controlling the layout
- Adds tests for  this layout and some mocks for testing layouts generally
- Improves the rendering of the reaction buttons which were not pixel perfect
- Adds the UI for the expand collapse buttons including the count of hidden items in the collapsed state.

* Add comment for reactionsCollapsed binding.

* Add the add more button and change the layout to be reactions specific and account for the add more button specifics.

* Update CollapsibleReactionLayout.swift

* Rename lines to rows, to hopefully ease the incoming merge.

* Add LayoutValueKey to organise subviews by item type.

- create ReactionLayoutItemType to tag subviews with ReactionLayoutItem
- Add LayoutValueKey support to the layout mocks
- Fix some mistakes in the merge
- Make the test code a bit more DRY
- Make the animation of the addMore and collapse buttons consistent

* Fix naming of the add more button label

* Address comments and fix padding

- Fix typos
- Use pdf/svg for icon
- Fix incorrect padding from a bad merge

* Fix "Add more" button padding to match designs

* Update snapshot test reference images.

* Fix naming, add Preserves Vector Data

ElementX/Sources/Generated/Assets.swift

@@ -42,6 +42,7 @@ internal enum Asset {
     internal static let locationPointerFull = ImageAsset(name: "images/location-pointer-full")
     internal static let locationPointer = ImageAsset(name: "images/location-pointer")
     internal static let timelineComposerSendMessage = ImageAsset(name: "images/timeline-composer-send-message")
+    internal static let timelineReactionAddMore = ImageAsset(name: "images/timeline-reaction-add-more")
     internal static let waitingGradient = ImageAsset(name: "images/waiting-gradient")
   }
 }