BP Signups WP List Tables need to have a column_default method

[imath]

In BP Signups WP List Tables, there are two filters to add custom column headers to the table :

• In mulitisite : 'bp_members_ms_signup_columns'
• In non multisite : 'bp_members_signup_columns'

If a plugin is adding a new column header for instance, we get this error:

Fatal error: Maximum function nesting level of '100' reached, aborting!

I suggest to add the column_default() method to BP_Members_List_Table & BP_Members_MS_List_Table to avoid it.

[imath]

In 8472:

Allow plugins to add a custom column in Signup WP_List_Tables

Thanks to a new "column_default()" function in "BP_Members_List_Table" and in "BP_Members_MS_List_Table" (multisite), plugins will be able to add data rows to the eventual custom columns they created using the filters "bp_members_signup_columns" and "bp_members_ms_signup_columns".

Fixes #5678