App polish part 2 (#1118)

- Fix sheet backgrounds in dark mode.
- Fix background colours for reaction buttons.
- Reply view layout.
- Update separator colours.
- Don't show sender name/avatar in DMs.
- Fix inconsistent line heights with formatted body text.
- Use plain text for reply
- Increase tappable size of collapsed state changes button.
- Blockquote layout and text colour.
- Tap to expand the topic in room details.
- Change the topic and security font size in room details.
- Add cancel button when inviting someone to an existing room.
- Reword Add Friends to Add People in start chat screen.
- Update compound.

ElementX/Sources/Screens/InviteUsersScreen/InviteUsersScreenModels.swift

@@ -22,6 +22,7 @@ enum InviteUsersScreenErrorType: Error {
 }
 
 enum InviteUsersScreenViewModelAction {
+    case cancel
     case proceed
     case invite(users: [String])
     case toggleUser(UserProfileProxy)
@@ -86,6 +87,7 @@ struct InviteUsersScreenViewStateBindings {
 }
 
 enum InviteUsersScreenViewAction {
+    case cancel
     case proceed
     case toggleUser(UserProfileProxy)
 }