Customizable KPI notification template

After #468 (closed) we wish to support template customization.

Templates are stored in AppResources. ApplicationStartup should add kpi-execution-template.mustache as one of dynamic liquibase changesets. Then we can load it with:

var template = appResourceService.getResource(AppResourceService.APP_NAME_GG_CE, "kpi.execution.template", Locale.getDefault());
if (template == null) { // Use resource

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