Resolve "Error 500 on route "/admin/application_settings/services/1882/edit" -> edit Prometheus Service Template"

This was caused by the element getting cached on the class before the
Vue app has finished rendering.

Closes #43560

Instead of converting hash into JSON inside
issuable_initial_data method, return hash and convert
to JSON later. This allows us to easily extend basic
issuable data with resource specific values. For example
for Epic these data should include also labels, so we can then
do something like:
issuable_initial_data(@epic).merge(labels: @epic.labels).to_json

This reverts commit 34e16110.

This reverts commit 28c6a6b0.