Allow users to update Upgrade Window Start

[mcbattirola]

This PR adds a prop children to the "Help & Support" page that allows enterprise to extend it. webapps.e will have its own page that will add a new section to Support with some cloud specific information. Some components of Support have been made more generic and exported so we can use them on enterprise.

This PR also updates the events table to properly show the new audit event type.

Enterprise PR using this feature: https://github.com/gravitational/webapps.e/pull/357

Testing

To run the whole thing, you can:

1. Run salescenter locally (follow this)
2. Checkout master mcbattirola/add-cloud-upgrade-windows on both teleport and teleport.e, and webapps.e on branch mcbattirola/add-cloud-upgrade-windows
3. comment this line on lib/modules/modules.go so we can run cloud without second factor
4. build and run teleport.e with this license (under auth_service in the config file). I recommend you copy cloud/dev/teleport/config.yaml and add:

auth_service:
  enabled: "yes"
  listen_addr: 0.0.0.0:3025
  tokens:
  - proxy,node:[token]
  authentication:
  license_file: /home/matheus/tc/db/fixtures/license.pem

You have to run it like this:

TELEPORT_CLOUD_HOSTPORT="api.localhost:3080" DEBUG=1 /path/to/teleport/e/build/teleport start -c my-config-file.yaml -d --insecure

6. Run this branch of the teleport UI with yarn start-teleport-e --target=https://localhost:4080/web
   Then you should be able to make a request from the local teleport to the salescenter

Screenshots

Closes https://github.com/gravitational/cloud/issues/1808

[mcbattirola]

Thank you @kimlisa !

I moved the upgrade window stuff to enterprise (PR here), now this PR only adds the event and adds a children prop to Support. I've updated the PR text to reflect this

[mcbattirola]

@ryanclark Thanks for the review, are you ok with merging this?

[ryanclark]

LGTM!