Mattermost-server: [Help Wanted] [PLT-5100] Add members count for the direct messages list

[esethna]

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Notes: Jira ticket

Add a count displaying the number of members that are displayed in the direct messages more modal.

Note that this should include only the members shown in the list, and should update depending on the filter chosen, and if there's a search done for a member.

Mobile view:

Desktop view:

[jsfeb26]

I'll take a shot at this one

[lisakycho]

I'll take this one!