Change `memberType` that refers to the model to `memberModelType`

What does this MR do and why?

In the app/assets/javascripts/members there are two different types of references to memberType which gets confusing.

The first memberType (the one we are keeping for now) is responsible for splitting up the members into different tabs and can have 4 values: user, group, invite, and accessRequest. This naming doesn't 100% make sense but will be a very big effort to change so we are leaving it for now.

The tabs

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The second memberType refers to the model type of the member. A member can either be a app/models/members/group_member.rb or a app/models/members/project_member.rb. On the frontend these are referred to as GroupMember or ProjectMember. This distinction is used in app/assets/javascripts/members/components/modals/remove_member_modal.vue#L113 to determine if the Also remove direct user membership from subgroups and projects checkbox should be shown.

To avoid confusion we are changing the second memberType reference to memberModelType. This MR also moves GroupMember or ProjectMember into constants.

Screenshots or screen recordings

No visual changes.

How to set up and validate locally

1. Navigate to a group -> Group information -> Members.
2. From the dropdown next to a member choose Remove member
3. Verify that the Also remove direct user membership from subgroups and projects checkbox is there.

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.