Audit event streaming Admin tab show in Premium license

Summary

Audit event streaming is an Ultimate feature. Currently the Streams tab is visible when a Premium license is applied. This tab should not show for Premium license.

This is a UI bug, if we actually try to add a stream we get a generic error.

Steps to reproduce

Have an instance with Premium license and access to Admin area. Go to Admin Area > Monitoring > Audit Events and look for the Streams

Example Project

What is the current bug behavior?

Stream Tab shows for Premium license

What is the expected correct behavior?

Relevant logs and/or screenshots

Stream Tab should only show for Ultimate license

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info

(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)

Possible fixes

I believe the issue comes from this line. Instead of returning true we should check if feature is available with License.feature_available?(:external_audit_events)