Activity Administration: add some hooks to help plugins developers

[imath]

Working on the BP Attachments plugin, i've found it was pretty hard to extend the Activity Administration so that you can easily :

• add extra content to the content of the activity see ​this comment
• add a new metabox see ​this comment
• or load some javascript/css see ​this comment

And i think we should enable plugins to add custom columns, just like it's the case for the Groups component.

See attached patch

[boonebgorges]

+1. Looks good to me.

[imath]

In 10093:

Activity Administration screens: Include new hooks to ease their customization

• the action bp_activity_admin_meta_boxes can be used to add new metaboxes when editing an activity from the corresponding Administration screen.
• the action bp_activity_admin_enqueue_scripts can be used to enqueue custom script in Activity Administration screens.
• the filter bp_activity_admin_comment_content can be used to add custom content to the activity content when viewing activities from the corresponding Administration screen.
• It is now possible to add custom columns and their corresponding rows in BP_Activity_List_Table

Fixes #6611