BuddyPress: Saving a group's settings wipes out forum tab

[r-a-y]

If you have a bbPress forum enabled in a BuddyPress group and attempt to save the BP group's admin settings, the forum tab will be removed.

Steps to reproduce:

1. Make sure you have a BP group with forums enabled
2. Navigate to the group's "Admin > Settings" page and click "Save".
3. The group's "Forums" tab is now removed

This is due to BP explicitly looking for a 'group-show-forum' POST value in the group admin settings page:
​https://buddypress.trac.wordpress.org/browser/tags/1.7.2/bp-groups/bp-groups-screens.php#L556

Because this value does not exist, the forum tab will be removed.

The easiest way to fix this is to add a hidden input 'group-show-forum' field on the group settings page.

[johnjamesjacoby]

In 5027:

Introduce group_settings_hidden_field() method to prevent the settings page from accidentally disabling the group forum. Props r-a-y. Fixes #2339.