Fix access level when searching by Epic + any other work item type

Problem description

Relates to this comment !195124 (comment 2599583972)

When doing a type search from GLQL like this one, we get wrong results from ES:

```glql
display: table
fields: title, labels("workflow::*"), author, labels
query: group = "gitlab-org" and type in (Epic, Task)
```

It happens because Epics have different access rules as they are related to groups only. As a result, the epic query use a different authorization and confidentiality filter.

Solution

• implement new scope: work_items with existing code & consume the new scope in GLQL finder (switch from issues to work_items)

• implement ability for work_items to query by EPIC type