Create new model for instance integrations

In order to support integrations in Cells architecture we need to split existing integrations into group/project integrations (existing integrations table) and instance wide integrations. Instance wide integrations need a separate table, which will be done in #474808 (closed)

However, to support the new table we also need to define a new InstanceIntegration model, which is identical to Integration model, with the exception of group/project association.

Proposed solution

Create a new InstanceIntegration model which is identical to InstanceIntegration model.

Perhaps this and #474808 (closed) issues need to be combined in one in order to ensure nothing breaks.

Additionally, create corresponding instance models for existing integrations (e.g. Integrations::Telegram should have a corresponding Integrations::Instance::Telegram). This is needed because instance_integrations has type STI column.