/json/users: Use field.realm for realm lookup.
/json/users: Use field.realm for realm lookup.

This avoids an unnecessary join to UserProfile.

To verify this, you can do `print(queries)` in the
`test_get_custom_profile_fields_from_api` test.  It's
kinda noisy, so I excerpted them below...

Before:

    SELECT ...
    FROM "zerver_customprofilefieldvalue"
    INNER JOIN "zerver_userprofile" ON ("zerver_customprofilefieldvalue"."user_profile_id" = "zerver_userprofile"."id")
    INNER JOIN "zerver_customprofilefield" ON ("zerver_customprofilefieldvalue"."field_id" = "zerver_customprofilefield"."id")
    WHERE "zerver_userprofile"."realm_id" = 2

After:

    SELECT ...
    FROM "zerver_customprofilefieldvalue"
    INNER JOIN "zerver_customprofilefield" ON ("zerver_customprofilefieldvalue"."field_id" = "zerver_customprofilefield"."id")
    WHERE "zerver_customprofilefield"."realm_id" = 2'

I don't have any way to measure the two queries with
realistic data, but I would assume the second
query is significantly faster on most of our instances,
since CustomProfileField should be tiny.

punchagan/zulip,synicalsyntax/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,brainwane/zulip,andersk/zulip,shubhamdhama/zulip,timabbott/zulip,brainwane/zulip,eeshangarg/zulip,zulip/zulip,synicalsyntax/zulip,timabbott/zulip,timabbott/zulip,shubhamdhama/zulip,rht/zulip,shubhamdhama/zulip,zulip/zulip,rht/zulip,rht/zulip,punchagan/zulip,eeshangarg/zulip,showell/zulip,brainwane/zulip,synicalsyntax/zulip,andersk/zulip,brainwane/zulip,zulip/zulip,brainwane/zulip,punchagan/zulip,synicalsyntax/zulip,punchagan/zulip,rht/zulip,andersk/zulip,timabbott/zulip,andersk/zulip,showell/zulip,kou/zulip,synicalsyntax/zulip,hackerkid/zulip,brainwane/zulip,synicalsyntax/zulip,hackerkid/zulip,shubhamdhama/zulip,zulip/zulip,shubhamdhama/zulip,rht/zulip,brainwane/zulip,eeshangarg/zulip,kou/zulip,timabbott/zulip,shubhamdhama/zulip,shubhamdhama/zulip,kou/zulip,showell/zulip,rht/zulip,showell/zulip,timabbott/zulip,punchagan/zulip,timabbott/zulip,punchagan/zulip,punchagan/zulip,kou/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,showell/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,eeshangarg/zulip,zulip/zulip,kou/zulip,andersk/zulip,showell/zulip,zulip/zulip,synicalsyntax/zulip,hackerkid/zulip,hackerkid/zulip,kou/zulip,showell/zulip,andersk/zulip,eeshangarg/zulip