[ticket/8309] Renaming "Last visit" to "Last activity"

PHPBB3-8309
[ticket/8309] Rename $last_visit to $last_activity

PHPBB3-8309
[ticket/8309] Moving key to get keys in alphabetical order

PHPBB3-8309
[ticket/8309] Removing key with similar function to avoid redundancy

PHPBB3-8309
[ticket/8309] Changing $last_activity to $last_active

PHPBB3-8309
Merge pull request #2137 from geetakshi/ticket/8309

[ticket/8309] Rename "Last visited" to "Last activity" on memberlist/viewprofile