Add a count for recently added group members

What does this MR do?

This MR is one of the many MRs to build the backend for Group Level Activity Analytics MVC

This MR adds a count for recently added group members, but only those that are visible for the user.

Database review notes

SQL

SELECT
    COUNT(*)
FROM ((
        SELECT
            "members".*
        FROM
            "members"
        WHERE
            "members"."type" = 'GroupMember'
            AND "members"."source_id" = 9970
            AND "members"."source_type" = 'Namespace'
            AND "members"."requested_at" IS NULL)
    UNION (
        SELECT
            "members".*
        FROM
            "members"
        WHERE
            "members"."type" = 'GroupMember'
            AND "members"."source_type" = 'Namespace'
            AND "members"."requested_at" IS NULL
            AND "members"."source_id" IN ( WITH RECURSIVE "base_and_descendants" AS (
(
                        SELECT
                            "namespaces".*
                        FROM
                            "namespaces"
                        WHERE
                            "namespaces"."type" = 'Group'
                            AND "namespaces"."parent_id" = 9970)
                    UNION (
                        SELECT
                            "namespaces".*
                        FROM
                            "namespaces",
                            "base_and_descendants"
                        WHERE
                            "namespaces"."type" = 'Group'
                            AND "namespaces"."parent_id" = "base_and_descendants"."id"))
                SELECT
                    "id"
                FROM
                    "base_and_descendants" AS "namespaces")
                AND "members"."user_id" NOT IN (
                    SELECT
                        "users"."id"
                    FROM
                        "users"
                        INNER JOIN "members" ON "users"."id" = "members"."user_id"
                    WHERE
                        "members"."type" = 'GroupMember'
                        AND "members"."source_type" = 'Namespace'
                        AND "members"."source_id" = 9970
                        AND "members"."source_type" = 'Namespace'
                        AND "members"."requested_at" IS NULL))) members
WHERE
    "members"."type" = 'GroupMember'
    AND "members"."source_type" = 'Namespace'
    AND "members"."created_at" >= '2019-12-26 12:35:40.208911'

Link to query run in #database-lab: https://gitlab.slack.com/archives/CLJMDRD8C/p1585144351110500

Query plan: https://explain.depesz.com/s/7Fl6

Summary

``` Time: 15.042 ms - planning: 1.972 ms - execution: 13.070 ms - I/O read: 0.000 ms - I/O write: 0.000 ms

Shared buffers:

• hits: 8467 (~66.10 MiB) from the buffer pool
• reads: 0 from the OS file cache, including disk I/O
• dirtied: 0
• writes: 0

  </details>

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