Revert, Follow-up: -1 recent contributors

Revert, Follow-up: -1 recent contributors

This reverts commit b67f40e5b01af569794eba26da79497329590d36.

There was an issue that the web installer, when inserting initial
values into the db table site_stats, would not insert any value for
ss_active_users. That lead to some places, most notably
Special:CreateAccount, to show off -1 recent contributors.

The commit to revert solved this by setting a default value of 1 for
the ss_active_users field in the database schema. However, while the
default schema has been changed, there hasn't been any schema update
procedure implemented. Also, the default of 1 is inconsistent with
other defaults in that table.

Revert that schema change completely. Instead, make the web installer
insert 0 to the field ss_active_users (just like it does with all other
fields in that table).

Bug: T56888
Change-Id: I4298754c2238f6e5447a5df77a4ada722ba302b2