Mattermost-server: Migrate 'components/post_view/new_message_seperator' module and associated tests to TypeScript

[mattermod]

We are starting to migrate the mattermost-webapp over to TypeScript to facilitate better code quality. This Help Wanted issue is to modify all files in components/post*view/new_message_seperator (not sub-directories included) and associated test files.

Below is a checklist of items you should be doing to migrate:

• < ] Rename the files to their associated TypeScript extensions (ie. js to ts, jsx to tsx)
• [ ] Update any components using those to the correct imports
• [ > Fix any errors generated by the TypeScript compiler. You can run make check-types to display any errors.

We are aiming to be as strict as possible for the time being, but if you run into any issues where the file cannot be successfully migrated without significant change in other libraries, or our tsconfig.json, please feel free to raise them as we are aiming to keep this process smooth and gradual, while enforcing as much as we can.

Some examples of already migrated files can be found here:

• https://github.com/mattermost/mattermost-webapp/pull/3790
• https://github.com/mattermost/mattermost-webapp/pull/3472
• https://github.com/mattermost/mattermost-webapp/pull/4230

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If you're interested please comment here and come join our "Contributors" community channel on our daily build server, where you can discuss questions with community members and the Mattermost core team. For technical advice or questions, please join our "Developers" community channel.

New contributors please see our Developer's Guide.

JIRA: https://mattermost.atlassian.net/browse/MM-20480

[abdusabri]

I can pick this one up.

@devinbinnie the component should be new_message_separator, not new_messages_below as written in the issue title and description, right?

[devinbinnie]

Huh, this is weird. Apparently new_messages_below was removed at some point, so we won't need to convert it to TypeScript.
BUT, there's no ticket anywhere for new_message_seperator...
Not sure how that happened.

Anyways, @abdusabri you can definitely work on new_message_seperator. I'll rename this and the JIRA ticket to fit.