PLT-3427 Added channel header to view channel info modal

[DavidLu1997]

Summary

Added channel header (with markdown formatting) to the view channel info modal.
Also removed Purpose and Header displays if they do not exist.

Ticket Link

https://mattermost.atlassian.net/browse/PLT-3427
#3563 (discussion)

Checklist

[Place an '[x]' (no spaces) in all applicable fields.]

• Added or updated unit tests (required for all new features)
• Added API documentation (required for all new APIs)
• Has driver changes that have been merged and package.json updated
• Has enterprise changes (please link)
• Has UI changes
• Includes text changes and localization files updated
• Touches critical sections of the codebase (auth, upgrade, etc.)

[enahum]

+1

Added label on PM Approval cause of discussion in #3534

[crspeller]

+1

[mattermod]

Spinmint test server being created...

[mattermod]

Spinmint test server created.

Link: http://i-319461a5.spinmint.com:8065/pr3584

Email: test@test.com

Email2: test2@test.com

Password: passwd

Instance ID: i-319461a5

[esethna]

Hey @DavidLu1997, tested, looks good, I I just want to make sure that this change does not cause previously set channel headers to be reset?

Tested in private groups and public channels:

1. Long headers (view info modal will scroll)
2. adding and removing headers and purpose
3. links, headings, bold, numbers, lists, inline images

[mattermod]

Spinmint test server destroyed

[DavidLu1997]

@esethna this PR does not affect previously set channel headers to reset, it only displays them.

[esethna]

Great, thanks!