Separate actions and screens in Messages component

[johnjamesjacoby]

Two screen functions also contain actions:

• messages_screen_compose() sends a private message
• messages_screen_notices() sends notices

[johnjamesjacoby]

The actionable functionality in these components should be moved into bp-messages-actions.php and hooked to bp_actions.

[johnjamesjacoby]

In 9947:

Messages: Separate actions from screen functions

This change extracts action-specific functionality from 2 message specific screen functions (per the component coding standards) by introducing two new functions in bp-messages-actions.php and hooking them to bp_actions. These new functions handle the creation of new private messages and notices, and the editing & deletion of existing notices.

A few additional improvements were also made to these functionalities:

• String improvements to provide additional clarity to site members performing message related actions
• Code scrutinization and documentation improvements to better understand what these functions are for and when they were introduced
• Updates to new helper functions introduced since these functions were first introduced and last updated

Fixes #6505 (in trunk, for 2.4)

[r-a-y]

In 9966:

Messages: Add phpDoc clarification about message action functions.

See #6505.