[Stack Monitoring][Angular removal] Add alerts to react app

[estermv]

Add all alerts related features to the react app for stack monitoring:

• Modal is shown when entering a page.
• Navbar button
• Badge in setup mode to configure alerts
• Badge that appears when an alert is active

Technical notes

• There is already a context that that shares the alerts with react child components.
• Alerts are fetched in the base_controller when a flag should_fetch is set to true from child controllers (like happens here)

As a first implementation, we can have a similar approach and add the same provider to the page_template and receive a boolean prop to indicate when to fetch alerts.

Note that alerts are fetched in each refresh interval, so the request needs to be added to the onRefresh callback in the page_template

[elasticmachine]

Pinging @elastic/logs-metrics-ui (Team:logs-metrics-ui)