Scaling of user statistics (#8277)

Fix https://github.com/FreshRSS/FreshRSS/issues/8268
To better support user management on FreshRSS instance with many users.

SQL speed improved. On a reduced test with 5 users, including some large accounts (PostgreSQL on a very tiny and slow server), improving from ~2.3s to ~1.8s, which gives ~20% speed improvement.

Then tested with 1000 users, with only the default feed (on my old desktop computer):

```sh
for i in {1..1000}; do ./cli/create-user.php --user=freshrss$i --password=freshrss; done
app/actualize_script.php
cli/access-permissions.sh
```

SQLite:

```console
$ time cli/user-info.php | wc -l
1001

real    0m1.366s
user    0m0.908s
sys     0m0.475s
```

PostgreSQL:

```console
$ time cli/user-info.php | wc -l
1001

real    0m28.498s
user    0m12.137s
sys     0m2.217s
```

MariaDB:

```console
# time ./cli/user-info.php | wc -l
1001

real    0m49.485s
user    0m1.276s
sys     0m2.258s
```

Yes, SQLite is much faster - not a surprise for such use-cases, where the TCP connection is not re-used.

I have added some CLI options to disable some statistics:

```sh
cli/user-info.php --no-db-size --no-db-counts
```

For the Web UI, I have disabled detailed user statistics if it takes too long, and retrieve missing user statistics asynchronously via JavaScript. Lazy loading of the user details based on IntersectionObserver, with maximum 10 requests in parallel.
Web UI tested on 1000 users as well. Checked with SeaMonkey.

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