Fix mobile dm compose

[brianlovin]

Status

• WIP
• Ready for review
• Needs testing

Deploy after merge (delete what needn't be deployed)

• hyperion (frontend)

Closes #4743

This fixes a nasty bug where DMing someone from their user profile on mobile would not actually take you to the new message composer screen. The root cause of this issue was that we added a /messages/new route into src/routes.js - in the DM view, we were checking for match.params.threadId and seeing if threadId === 'new' to determine if we should show the composer or not. But since /messages/new does not pass any match.params, this was failing.

I've cleaned up that logic by removing the unneeded route entirely. @mxstbr a future iteration here - which isn't high priority right now - is to support /messages/new as a modal route so that you can initiate a DM from anywhere in the app without having to have a whole redirect to /messages

[spectrum-bot]

	Warnings
⚠️	These modified files do not have Flow enabled: src/views/titlebar/index.js

Generated by 🚫 dangerJS

[brianlovin]

Doing a prod cut this morning, going to admin merge. Open to any follow-on feedback.