Mono repo -> Master (#22553)

Combines the following repositories into one:

https://github.com/mattermost/mattermost-server
https://github.com/mattermost/mattermost-webapp
https://github.com/mattermost/focalboard
https://github.com/mattermost/mattermost-plugin-playbooks

server/tests/test-link-preview.md

@@ -0,0 +1,53 @@
+# Link Preview Tests
+
+Link previews should embed previews of the contents of a hyperlink from a message or comment in the center channel directly below the message or comment.
+
+Post location variation: 
+
+1. Post message in center channel with RHS closed (Expected: preview of first link renders under message) 
+2. Post message in center channel with RHS open (Expected: preview of first link renders under message) 
+3. Post comment in RHS (Expected: link preview does not render) 
+4. View comment in center channel with RHS closed (Expected: preview of first link renders under message) 
+5. View comment in center channel with RHS open (Expected: preview of first link renders under message) 
+6. Search for post in RHS with link (Expected: no previews render in search results) 
+
+Test the above variations with the below sample messages (e.g. 1-A, 2-B, 3-C, 4-D, etc.) 
+
+```
+#### A) No link preview
+
+Link 1 example: https://en.wikipedia.org/wiki/Olympus_Mons
+
+Link 2 example: http://www.techmeme.com/
+
+Link 3 example: https://about.gitlab.com/
+```
+
+```
+#### B) Link preview without an image
+
+Link 1 example: https://coveralls.io/builds/9818822/source?filename=app%2Faudit.go
+```
+
+```
+#### C) Link preview with image in top right corner
+
+Link 1 example: http://www.theglobeandmail.com/news/national/three-canadians-shortlisted-for-global-teacher-prize/article33429901/
+
+Link 2 example: https://twitter.com/ArchieComics/status/813007703861841920
+
+Link 3 example: https://github.com/mattermost
+
+Link 4 example: http://stackoverflow.com/questions/36650437/using-mattermost-api-via-gitlab-oauth-as-an-end-user-with-username-and-password
+```
+
+```
+#### D) Link preview with image at the bottom
+
+If "Account Settings > Display > Default appearance of image link previews" is set to "Collapsed", you must click the expand arrows to display the image.
+
+Link 1 example: https://www.yahoo.com/news/panasonic-unveils-solar-roof-may-212400917.html
+
+Link 2 example: https://mattermost.com
+
+```