Cannot configure legacy group forums

[r-a-y]

Original report:
http://buddypress.org/support/topic/bp-1-7-1-and-the-forums-component-has-not-been-set-up-yet-error/

In BP 1.7, if the "Group Forums (Legacy)" component is enabled, the "Forums" tab (which is used to setup the forums) does not show up.

We need to enable the tab for sites that are still using the legacy forums.

Attached patch:

• Reinstates the "Forums" tab in the admin area if the legacy group forums component is toggled.
• Properly deprecate forum code between 1.6 and 1.7. (Was running into function already declared errors.)
• Loads the deprecated forum code in the admin area only.
• Change the strings on the "Forums" tab to note that bbPress is the preferred option going forward.

Feel free to change the strings.

[boonebgorges]

In 7213:

Reintroduces legacy forum config tab for installations still using legacy forums

Without access to this page, it becomes impossible to regenerate the
bb-config.php file. We also lose the chance to preach the gospel of the new
bbPress plugin to users of bbPress 1.x when we don't have an admin panel for
the legacy component.

Fixes #4999

Props r-a-y