Remove the notion of sending messages from the activity macro

XWIKI-7679 began with extracting the message sending functionality from the activity macro as a reusable macro called "messageSender". This particular issue is more for convenience and aims at doing what the original idea of XWIKI-7679 suggested, namely to completely remove the message sending functionality from the activity macro.

This way we have 2 clear macros, no more mixing:

• messageSender for sending messages
• activity for displaying messages (and other events)

The Dashboard and the user profile will have a new dedicated section for sending messages (that uses the messageSender macro).

Additionally, it makes it easier for users to remove the ability to send messages from the homepage by editing the Dahsboard and removing the "Send Message" gadget.