[OPT-6670] Create instant preview dashboard upon importing a process definition (backend)

Create a new "instant-preview" dashboard whenever a process definition is imported. Characteristics of this dashboard:

Must have a predictable URL based on the process definition key
The dashboard template is a JSON file with a fixed name (location and name TBD). Depending on the query parameter values, a new template can be used. Like this we can have several templates
The Backend will parse the Json file (which will be a file generated from Optimize using the export API (see below). Since the exported file will have concrete IDs (for Dashboards/Reports/XML Diagrams, etc), the backend needs to parse this file and replace the concrete data by placeholders, so that the dashboard can be used as a template for other processes. This makes it easier for the product manager to define and export a new dashboard template
```
GET /api/export/dashboard/json/{dashboardId}/{fileName} 
```

~~Make sure that access to this dashboard is only granted to users with proper permission~~: Will be handled in ~~OPT-6701~~
The request to the back end has the form:

GET /api/dashboard/instant/<PROC_DEF_KEY>/?template=<JSON_FILE_NAME>

The response of this request is still to be defined, but it will be analogous to

GET /api/dashboard/:id

Generation and update of the instant preview dashboards will be handled in ~~OPT-6675~~

Testing notes

Can only be tested in conjunction with ~~OPT-6675~~, so testing notes will be written there

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