Allow filtering by group in epic list and roadmaps
## Release Post
We have expanded [epic list filters](https://docs.gitlab.com/ee/user/group/epics/manage_epics.html#filter-the-list-of-epics) and [roadmap filters](https://docs.gitlab.com/ee/user/group/roadmap/#sort-and-filter-the-roadmap) to include the ability to filter by a specific group, providing users with more flexibility and efficiency in targeting the information they need.
## Problem to solve
In the words of an actual customer:
> As a large organisation, some of our most important epics are quite high up the Group structure (i.e. they are on a group with a LOT of sub-groups...1000s!) Often, we want to find epics which are created just on that high-up group level, ignoring all the subgroups (where there are hundreds of other sub-epics with similar labels/names etc). Could we add a flag/toggle on the Epic List page to allow you to show items JUST on that group, or to INCLUDE ALL SUBGROUPS? Something similar on the Roadmap tab would be great, but the list view is where we are currently feeling the most pain
## Proposal
Create a `group` filter option in the epic list which allows the user to filter search results by the immediate group.
Consider the following example:
Group structure:
1. ABC Company (parent group)
1. XYZ (sub group)
2. 123 (sub group)
1. 987 (sub-sub-group)
3. lmn (sub group)
Example search filters:
1. When in the parent group, `ABC Company`, on the epic list, add `group::ABC Company` to filter to only display results for that group.
1. While in the same location, have the ability to filter for just a specific subgroup like `group::123`.
2. While in the sub group `123`, on the epic list, add `987` to filter to only display results for the sub-sub-group `987`.
## Customer Feedback on Proposal
>An example use case it would cover is I'm a dept head, so I'm on my group and my 10 teams are all sub groups. I'm looking at my epics for the whole dept, and I can also easily filter on one team at a time to see what each is doing, or I can filter on just the epics at the dept level (i.e. just the big stuff)
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