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

webapp/channels/.github/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,72 @@
+<!-- Thank you for contributing a pull request! Here are a few tips to help you:
+
+1. If this is your first contribution, make sure you've read the Contribution Checklist https://developers.mattermost.com/contribute/getting-started/contribution-checklist/
+2. Read our blog post about "Submitting Great PRs" https://developers.mattermost.com/blog/2019-01-24-submitting-great-prs
+3. Take a look at other repository specific documentation at https://developers.mattermost.com/contribute
+-->
+
+#### Summary
+<!--
+A description of what this pull request does.
+-->
+
+#### Ticket Link
+<!--
+If this pull request addresses a Help Wanted ticket, please link the relevant GitHub issue, e.g.
+
+  Fixes https://github.com/mattermost/mattermost-server/issues/XXXXX
+
+Otherwise, link the JIRA ticket.
+-->
+
+#### Related Pull Requests
+<!--
+List all PRs related to resolving a ticket. For instance, if you submitted a PR to `mattermost/mattermost-server`, please include it here.
+-->
+- Has server changes (please link here)
+- Has mobile changes (please link here)
+
+#### Screenshots
+<!--
+If the PR includes UI changes, include screenshots/GIFs.
+
+For an easier comparison of UI changes a table (template below) can be used.
+
+|  before  |  after  |
+|----|----|
+| <insert before screenshot here> | <insert after screenshot here> |
+
+-->
+
+#### Release Note
+<!--
+Add a release note for each of the following conditions:
+
+* Config changes (additions, deletions, updates)
+* API additions—new endpoint, new response fields, or newly accepted request parameters
+* Database changes (any)
+* Websocket additions or changes
+* Anything noteworthy to a Mattermost instance administrator (err on the side of over-communicating)
+* New features and improvements, including behavioural changes, UI changes and CLI changes
+* Bug fixes and fixes of previous known issues
+* Deprecation warnings, breaking changes, or compatibility notes
+
+If no release notes are required write NONE. Use past-tense. Newlines are stripped.
+
+Examples:
+
+```
+Added new API endpoints POST /api/v4/foo, GET api/v4/foo, and GET api/v4/foo/:foo_id.
+```
+
+```
+Added a new config setting ServiceSettings.FooBar. Added a new column Foo to the Users table.
+```
+
+```
+NONE
+```
+-->
+```release-note
+
+```