[ticket/11445] Optimize no of queries in get_global_subscriptions

PHPBB3-11445
[ticket/11445] Improve unit tests for get_global_subscriptions

PHPBB3-11445
[ticket/11445] Remove unused foreach key $group_name

PHPBB3-11445
[ticket/11445] Move get user's notification code into its own method

PHPBB3-11445
Merge pull request #2527 from dhruvgoel92/ticket/11445

[ticket/11445] optimize no of queries in get_global_subscriptions( )

* dhruvgoel92/ticket/11445:
  [ticket/11445] Move get user's notification code into its own method
  [ticket/11445] Remove unused foreach key $group_name
  [ticket/11445] Improve unit tests for get_global_subscriptions
  [ticket/11445] Optimize no of queries in get_global_subscriptions