Change isAdmin to isSchoolAdmin

[jankapunkt]

To comply with our roles system we need to change many isAdmin checks to isSchoolAdmin but keep in mind that school admins have still limitations:

schoolAdmin

• Can manage user accounts of the same institution (see scopes section)
• Can manage user roles (except admin) of the same institution
• Can view application logs
• Can change the theme for the same institution
• Can update system-wide ui settings for the same institution
• Can change the application logo for the same institution

admin

• can manage all user roles, even admin
• can manage legal documents and terms
• can apply settings for all institutions (as defaults)