webchat: replace window location on quick exit

[mythilytm]

Primary reviewer:

Description

• This replaces window location on quick exit instead of navigating so the back button won't work.
• Webchat was moved to flex before this change. This is an identical PR to chore: replace window location on quick exit webchat#276.

Checklist

• Corresponding issue has been opened
• New tests added
• Feature flags added
• Strings are localized
• Tested for chat contacts
• Tested for call contacts

Related Issues

Fixes # https://tech-matters.atlassian.net/browse/CHI-2204

Verification steps

• Ensure that Quick Exit button actually clears browsing history