Bump Web Chat to 4.15.1

[compulim]

Recently, Web Chat released 4.15.1. Most of the fixes are for accessibility issues.

I hand-picked a few PRs that Emulator team may want to look at:

• https://github.com/microsoft/BotFramework-WebChat/pull/4143
  • For OAuth card, the sign-in URL must be either http: or https:
• https://github.com/microsoft/BotFramework-WebChat/pull/4108, particularly
  • A new keyboard help screen
  • Focus trap on interactive element
• https://github.com/microsoft/BotFramework-WebChat/pull/4095
  • tl;dr Hello {1} should now render as Hello {1}, instead of previously Hello

[enhancement]