Activity dashboard - Add 'action' column

[r-a-y]

The activity dashboard is pretty cool, but could use an 'action' column to determine what type of activity the current row is.

[r-a-y]

In 8192:

Introduce bp_activity_admin_get_activity_actions().

This function walks through all currently registered activity actions
and returns a flattened array of the activity actions.

Format returned is:

Array(

'activity_update' => 'Posted a new status update',
...

)

See #5484

[r-a-y]

In 8193:

Activity dashboard: Add 'action' column.

This commit adds an 'Action' column to the activity dashboard. This
will give better context to the administrator when viewing the
dashboard.

Fixes #5484