Ineffective implementation of can_request_help in permissionsView

[zenovich]

#1006 implemented changes described in #1004 by making a separate DB query getting values of can_request_help and then one DB query for each row with can_request_help to determine if the group name can be shown to the user. So, "N+1 selects problem".

We need to rework it.

Ideas:

1. We could use JSON_ARRAYAGG to get values of can_request_help:

+------------------------------------+
| JSON_ARRAYAGG(can_request_help_to) |
+------------------------------------+
| [120505918794412400]               |
| [3, 3]                             |
| [3]                                |
+------------------------------------+
3 rows in set (0.00 sec)

2. We could make just one query for all the groups at once to figure out if a group is visible or not.

3. Probably we could even get all the info in just one query (a problem marked with a start 😉)

[zenovich]

@smadbe, @GeoffreyHuck