Create new function to wrap bp_get_group_description_excerpt()

[hnla]

Provide a nouveau wrapper function to allow a checks on group dir entry items layou:t i.e list or grid boxes.

This will add the ability to pass in a length value to further truncate the excerpts to prevent overlong desc breaking box heights when displaying in grid layouts.

This relates to #113

Relates to BP core ticket adding $length param desc excerpt:

https://buddypress.trac.wordpress.org/ticket/7493

[hnla]

N.B the approach changed as referencing bp_get_group_description_excerpt() was pointless instead like the core template tag we reference bp_create_excerpt() passing in our preferred $length values