Content safety

[enahum]

Summary

This PR adds some safety checks around post message attachments and app bindings to prevent the app from crashing when null is present in the post metadata for some reason.

Also includes a fix for YouTube video playback (for example this one https://www.youtube.com/watch?v=eWCXZIRm7iU&t=707). Fix taken from 0xced/XCDYouTubeKit#534

Release Note

* Prevented crash when message attachments or app bindings are `null`
* Fixed iOS YouTube video playback

[mattermod]

Building app in separate branch.

[lindalumitchell]

Verified on test builds on Android and iOS (13.7 on iPad and 14.3 on iPhone 7) that the YouTube link opens the video as expected. LGTM.

Note: Created https://mattermost.atlassian.net/browse/MM-36688 for the issue.

[mattermod]

Cherry pick is scheduled.

[mattermod]

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

+++ Updating remotes...
Fetching upstream
Failed to add the RSA host key for IP address '140.82.113.3' to the list of known hosts (/app/.ssh/known_hosts).
From github.com:mattermost/mattermost-mobile
 * [new branch]          MM-17135              -> upstream/MM-17135
 * [new branch]          MM-28880              -> upstream/MM-28880
 * [new branch]          MM-34619              -> upstream/MM-34619
 * [new branch]          build-pr-5486-54874f9 -> upstream/build-pr-5486-54874f9
 * [new branch]          build-pr-5489-65053bd -> upstream/build-pr-5489-65053bd
   cb7f97e88..47d081b21  master                -> upstream/master
 * [new branch]          onViewableItemsChange -> upstream/onViewableItemsChange
   2119cb4f8..b028b2e14  release-1.44          -> upstream/release-1.44
 + a8e37ed42...7142ec639 version               -> upstream/version  (forced update)
Fetching origin
Failed to add the RSA host key for IP address '140.82.113.3' to the list of known hosts (/app/.ssh/known_hosts).
+++ Updating remotes done...
+++ Creating local branch automated-cherry-pick-of-mattermost-mobile-#5486-upstream-release-1.44-1624645406
Switched to a new branch 'automated-cherry-pick-of-mattermost-mobile-#5486-upstream-release-1.44-1624645406'
Branch 'automated-cherry-pick-of-mattermost-mobile-#5486-upstream-release-1.44-1624645406' set up to track remote branch 'release-1.44' from 'upstream'.

+++ About to attempt cherry pick of PR #5486 with merge commit 47d081b216aab55bb81b0a92de6a233c99bdf26f.

error: could not apply 47d081b21... Content safety (#5486)
hint: after resolving the conflicts, mark the corrected paths
hint: with 'git add <paths>' or 'git rm <paths>'
hint: and commit the result with 'git commit'

+++ Conflicts detected:

UU ios/Podfile.lock
Aborting.

+++ Aborting in-progress git cherry-pick.

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

[enahum]

manual cherry-pick