[MM-402] Remove react-markdown package and use functions exposed by mattermost webapp

[ayusht2810]

Summary

• Removed react-markdown package and used functions exposed by mattermost webapp
• Converted index.js file to index.ts present in root of webapp folder

Screenshot

Before

After

Ticket Link

https://github.com/mattermost/mattermost-plugin-gitlab/security/dependabot/11

What to test?

• Complete test of link tooltip feature

Checklist

• [x] Completed dev testing
• [x] make test Ran test cases and ensured they are passing
• [x] make check-style Ran style check and ensured both webapp and server pass the checks

[ayusht2810]

@mickmister We usually assign PRs created by us to one of our team member as well

[ayusht2810]

@mickmister Should I move the PR for QA review?