Chore: Migrate AddExistingChannelView to Typescript

[reinaldonetof]

Proposed changes

Issue(s)

How to test or reproduce

• Create a Team, then click on "edit/create" icon on the header, and the new header title should be Add Channel to Team, so you need to click on Add Existing
• Then check if is working properly the query, search, and other functions.

Screenshots

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• Improvement (non-breaking change which improves a current function)
• New feature (non-breaking change which adds functionality)
• Documentation update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING doc
• I have signed the CLA
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works (if applicable)
• I have added necessary documentation (if applicable)
• Any dependent changes have been merged and published in downstream modules

Further comments

[AlexAlexandre]

LGTM :)