Show alerts tab in CCSM depending on UIConfiguration flag

Context:

In CCSM we have so far been hiding the alerts tab in collections. Once we add user search functionality and email server configuration we can show the alerts tab again. Can be implemented once OPT-7417 is done.

AT:

• functionality mirrors alerts in SaaS
• in CCSM, if the "userSearchAvailable" flag in the UIconfiguration is true, alerts tab in collections is shown again and new alerts can be configured
• in CCSM, if the "userSearchAvailable" flag in the UiConfig is false, the alerts tab should be hidden again 
• the BE validation of emails is shown in FE: if any of the emails configured in the alert cannot be found in identity, the BE returns a validation error which is displayed for the user

Testing Setup:
To test this, two things need configured: Email server in optimize config and additional users in keycloak/identity.
Email configuration: the Optimize configuration can basically be the same as described on our "test the alerting" confluence page (be connected to VPN and add email config to Optimize): https://confluence.camunda.com/display/CO/Test+the+Alerting 
Additonal users: Testing setup to configure users in CCSM described here: https://docs.google.com/document/d/1jQ49moF5KiMi3ZDSHfE3CKzObFyhAYbfZrvCqgujg44/edit?usp=sharing 

Test Cases:
In CCSM, when using identity with keycloak, the alerts tab in collections is visible again and alerts can be configured. When adding an email to an alert, only emails of users that exist in identity (/keycloak) can be added. If an email is added that does not exist in identity, an error message is shown and the alert cannot be configured. Alerts are sent out as expected. Test cases:

• in CCSM, alerts tab is visible
• email server is not configured --> info in UI that it needs configured to use alerts
• email server is configured + 1 email added to alert does not exist in keycloak --> error msg in UI that email doesnt exist, alert cannot be saved
• email server is configured + 1 email added to alert does not exist in keycloak and 1 email in alert does exist in keycloak --> error msg in UI that 1 email doesnt exist, alert cannot be saved
• email server is configured + email(s) added to alert that all exist in keycloak –> alert can be saved and is sent out to email as expected