Fix incorrect link handling of inline MD images

[abdusabri]

Summary

As of MM-12898, inline markdown images open the image preview modal, except if the markdown image itself is a link. e.g. [![Build Status](https://travis-ci.org/mattermost/platform.svg?branch=master)](https://travis-ci.org/mattermost/platform).

This PR fixes the following bug:

For a case like ![image](https://www.mattermost.org/wp-content/uploads/2016/03/logoHorizontal.png) an image plus some text that has [a link](http://somelink)

Observed behavior: The image doesn't have a pointer cursor, and doesn't open the preview modal

Expected behavior: The image should have a pointer cursor and open the preview modal

Reason of wrong behavior: just because there is a link ([a link](http://somelink)) in the markdown, it falsely assumed that the image itself is actually a link - which is not the case in the example given above

The bug is not a regression, it is a missed case since the implementation of MM-12898, but was only discovered after MM-15232, MM-18158. The issue can be reproduced on community V5.18 and latest master. The bug is not limited to channel header descriptions or its associated system messages, and can be reproduced in regular chat messages as well.

Ticket Link

https://mattermost.atlassian.net/browse/MM-21519

Screenshots

Before the fix

After the fix

[jgilliam17]

Thank you @abdusabri
Tested, looks good to merge.
Markdown has a pointer and opens in the preview as expected.

[mattermod]

Test server destroyed

[mattermod]

This issue has been automatically labelled "stale" because it hasn't had recent activity.
A core team member will check in on the status of the PR to help with questions.
Thank you for your contribution!

/cc @jasonblais @hanzei

[sudheerDev]

@abdusabri 👍 Thanks for the PR.

[sudheerDev]

/update-branch

[mattermod]

Will try to auto merge this PR once all tests and checks are passing. This might take up to an hour.

[amyblais]

This can be cherry-picked to v5.19 as long as it's not risky. @sudheerDev

[sudheerDev]

/cherry-pick release-5.19

[mattermod]

Error trying doing the automated Cherry picking. Please do this manually

+++ Updating remotes...
Fetching upstream
Fetching origin
+++ Updating remotes done...
+++ Creating local branch automated-cherry-pick-of-#4543-upstream-release-5.19-1578335234
Branch 'automated-cherry-pick-of-#4543-upstream-release-5.19-1578335234' set up to track remote branch 'release-5.19' from 'upstream'.
+++ Downloading patch to /tmp/4543.patch (in case you need to do this again)

+++ About to attempt cherry pick of PR. To reattempt:
  $ git am -3 /tmp/4543.patch

Applying: Fix incorrect link handling of inline MD images
Using index info to reconstruct a base tree...
M	utils/__snapshots__/message_html_to_component.test.jsx.snap
M	utils/message_html_to_component.jsx
M	utils/message_html_to_component.test.jsx
Falling back to patching base and 3-way merge...
Auto-merging utils/message_html_to_component.test.jsx
CONFLICT (content): Merge conflict in utils/message_html_to_component.test.jsx
Auto-merging utils/message_html_to_component.jsx
CONFLICT (content): Merge conflict in utils/message_html_to_component.jsx
Auto-merging utils/__snapshots__/message_html_to_component.test.jsx.snap
Patch failed at 0001 Fix incorrect link handling of inline MD images
Use 'git am --show-current-patch' to see the failed patch
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".

+++ Conflicts detected:

UU utils/message_html_to_component.jsx
UU utils/message_html_to_component.test.jsx

+++ Aborting in-progress git am.

+++ Returning you to the master branch and cleaning up.

[sudheerDev]

@amyblais will manually cherrypick