As a user I want to link an existing activity directly from the "related entities" table embedded in my school details view. Currently I can only create a new activity from the popup using the table's plus button. To link an existing one, I have to navigate to the relevant activity's details view and edit this through its entity-select field.

[actually, for activity-school relationships this is possible already but needs to be generalized for any two entity types]

Requirements:
for any RelatedEntitiesComponent:

• enter details in one or more fields (e.g. category + name) of the empty form and send to service that returns suggested similar existing records
• see if there are existing records and be able to select one to be selected, loading the existing into the form and applying any values needed to link it to the current context
• clearly indicate in the form whether you are creating a new record or editing an existing one
• develop filtering logic to find similar existing records (maybe based on datatype of the field, e.g. fuzzy match for "string", exact match for "enum"?)
• configure which fields "make sense" to be used for filtering the offered existing records
• remove EditTextWithAutocompleteComponent and replace it with the newly developed, more general component

Possible Approach
We already have a special feature for linking existing RecurringActivities in the ActivitiesOverviewComponent (in demo system: open School details > Activities tab > create new button > use "title" field to search for existing).
This is done using the EditTextWithAutocomplete component as a special form field. However, the current implementation requires the ActivitiesOverviewComponent to inject the id of the "parent" entity into the column config at runtime. Therefore, this currently can't be used in standard RelatedEntitiesComponent.

--> generalize this, so that we can simply use this editComponent on any field in a RelatedEntities table.
--> display as separate "field", like the "Currently: Active" indicator for School History entries?

this is the first case in the visualization (second case, see #2213)