fix(members): prevent members search with empty query
chore(twitter): cleans up Twitter API to work with 1.9 session

This updates the session handling to use the new elgg_get_session and set/get/remove
rather than the deprecated array access. Also cleans up ElggSession a bit.
fix(notifications): Corrected html entities handling for email subject and body

Fixes #6905
Merge pull request #6978 from jdalsem/members-search-blank

fix(members): prevent members search with empty query
Merge pull request #7016 from mrclay/6917

chore(twitter): cleans up Twitter API to work with 1.9 session
fix(groups): give feedback if a user cannot be added to a group

Fixes #6081
Merge pull request #7022 from beck24/6089-group-join-action-message

fix(groups): give feedback if a user cannot be added to a group
feature(discussions): Added email SMTP headers for better thread grouping.

Additionally changed message subjects for group discussions to make Gmail happy.

Fixes #6894
fix(release): Corrected release script Windows system compatibility
Merge pull request #6965 from Srokap/ticket_6909

fix(notifications): Better threading + resolve html entities decoding issue
chore(i18n): update translations
chore(release): v1.9.0-rc.4
Merge pull request #7028 from Srokap/release-1.9.0-rc.4

Release 1.9.0 rc.4
fix(river): add enabled col to river table, update on enable/disable of referenced entities

Fixes #6022 - empty river items fetched when referenced entities are disabled
Merge pull request #7013 from beck24/6022-empty-river

fix(river): add enabled col to river table, update on enable/disable of ...
fix(core): output/iframe made to the w3c standard

Not all browsers support the self closing iframe tag, according to the
w3c standard it's not self closing.
Merge pull request #7040 from jeabakker/iframe-w3c-standard

fix(core): output/iframe made to the w3c standard
fix(upgrade): use correct table prefixes on river upgrade script

Fixes #7033
Merge pull request #7053 from beck24/7033-dbprefix

fix(upgrade): use correct table prefixes on river upgrade script
chore(i18n): update translations