[Custom threshold] Simplify equation section

[maryam-saeidi]

Summary

Since for the equation, we have a label, we would like to simplify showing the equation with only showing the title in the first view and then allowing the user to expand/edit it if they want to.

This will be useful when prefilling the equation section by Host related equations such as CPU usage:

[elasticmachine]

Pinging @elastic/obs-ux-management-team (Team:obs-ux-management)

[jasonrhodes]

@maryam-saeidi whose input do we need to decide on this? @maciejforcone I assume?

[maryam-saeidi]

@jasonrhodes Yes, we need @maciejforcone 's input for this.