[Alerting] Adapt all existing applications to use the new KibanaThemeProvider

[mikecote]

See #118866

For the css-in-js initiative, the EUI team recently added a new theme context provider: EuiThemeProvider (see #108972 and elastic/eui#4511).

To have Kibana be ready to use the features related to this theme provider, we need all react applications to wrap their root component / react trees with it. Ideally, this would have been done exclusively at core's level, however, due to the agnostic nature of core's apps, and the fact that applications are effectively distinct react trees (e.g calling ReactDOM.render during their mount), we will need all application owners to adapt their application mounting points.

Also, applications are not the only parts of Kibana rendering react trees. For example, all usage of kibana_react's toMountPoint also need to be adapted, as, for example, when calling core.overlays.openFlyout(mountPoint). Management sections needs to be adapted too.

EUI is planning on leveraging the theme provider to carry the reset styles in their next phase, for 8.1, so this has to be done for the 8.1 FF

[elasticmachine]

Pinging @elastic/kibana-alerting-services (Team:Alerting Services)