feature(settings): adds setting for default number of items per page

Most queries that default to 10 now use the setting, whose valid range
is, for now, 2 to 5000. In order to keep this simple, queries that
default to 20 items will continue showing at least 20 items; the setting
is used only if it’s greater than 20.

Fixes #2650
fix(river): allow everyone to look at everyones activity page

fixes #6116
fix(db): elgg_get_metastring_id should always create an id

fixes #7579
Merge pull request #7616 from jdalsem/activity-for-everyone

fix(river): allow everyone to look at everyones activity page
chore(cache): simplifies MemoryPool by not proxying a Stash cache
Merge pull request #7604 from mrclay/lighter_pool

chore(cache): simplifies MemoryPool by not proxying a Stash cache
Merge pull request #7598 from mrclay/default_limit_110

feature(settings): adds setting for default number of items per page
Merge pull request #7590 from jdalsem/metastring-id-fix

fix(db): elgg_get_metastring_id should always create and id