Adds back-end support for restricting searches to specified context
ids (for example so it is possible to search only a specific forum).

Conflicts:
	mod/assign/styles.css

The new recordset support for Postgres requires transactions and
will cause errors if recordsets are not closed correctly. This
commit fixes problems that were identified during unit tests, and
via some basic code analysis, across all core code. Most of these
are incorrect usage of recordset (forgetting to close them).

On Postgres, get_recordset_sql loads all the results into memory
(within the Postgres library, which doesn't count towards the PHP
memory limit, but does count towards making your server run out of
memory) as soon as the query completes.

This commit changes the code to use cursors, which in Postgres
allow the results to be returned in smaller chunks (by default
100,000 rows).

Before MDL-59854 it was possible to have duplicate forum subscriptions.
Trying to import backups created from back then, caused a DB exception
due to unqiue key constraints. Now only one of multiple identical forum
subscritions is restored.

- If 'coursename' is specified in the CSV it should match the course short name - thanks Yusuf Yılmaz for the patch
- If 'idnumber' is specified but 'groupidnumber' is not, idnumber should be used for matching the course idnumber only
- If 'groupingname' is not specified, there should be no notices (regression from MDL-42514)
- If 'coursename' or 'idnumber' column is present, it can contain empty values in some/all lines

The function groups_get_user_groups is called too often both before rendering the page and after the page is rendered (using ajax).
The function was executing a query joining 3 tables in each call. The plementation of the function has now modified to store the
query result in a request cache.