Migrate "components/app.jsx" and tests to Typescript

[mattermod]

We are commencing the migration of the mattermost-server/webapp to TypeScript in order to enhance code quality. This "Help Wanted" issue pertains to the migration of the aforementioned "JSX" files and their associated test files to TypeScript.

Below is a checklist outlining the tasks to be performed during the migration:

< ] Rename the files to their corresponding TypeScript extensions (e.g., js to ts, jsx to tsx).
[ ] Update any components that utilize these files with the correct imports.
[ ] Migrate the associated tests, if applicable.
[ > Address any errors raised by the TypeScript compiler. You can execute make check-types to view any encountered errors.

Our objective is to maintain a high level of stringency during this process. However, if you encounter any obstacles where successful migration would necessitate significant changes to other libraries or our tsconfig.json file, please raise the issues. We aim to ensure a smooth and gradual migration while enforcing as many changes as possible.

For reference, you can find examples of already migrated files in the following pull requests:
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-52821

[tushar2708]

I would love to start working on this one.
I am new here, so have one question:
It seems that this change will spread across multiple files (mostly because of usage of changed variables), so will it be okay if the raised PR is a little big? Or would you expect something different?

[amyblais]

I am new here, so have one question: It seems that this change will spread across multiple files (mostly because of usage of changed variables), so will it be okay if the raised PR is a little big? Or would you expect something different?

cc @M-ZubairAhmed

[M-ZubairAhmed]

@tushar2708 thank you for coming forward to work on this.
Specific to this ticket I checked the usage of the above-mentioned file, And this is only imported in the entry file and is also used in the webapack config; so with that, the references to the file are only in two places. Please go ahead with the PR and we can continue the discussion there. :)