Add .clear_messages() method to django.contrib.messages

[benjaoming]

Disclaimer: This is really a minor issue 😊

We can do:

`
from django.contrib.messages import get_messages
`

And then we can call get_messages() - and if we iterate over them, they will be cleared.

But we can't explicitly clear them because this method doesn't exist:

`
from django.contrib.messages import clear_messages
`

For me, the use-case was test-related: Writing an e2e test case that calls some views and at each step verified messages, then cleared them.

Not sure if adding the method would make people use the messages framework wrongly, but I think it makes sense to note that ADDING clear_messages to the mix might make some people think you need to call it... which in most cases, you don't, and therefore the documentation needs to be clear (pun intended).

Forum discussion: ​https://forum.djangoproject.com/t/why-isnt-it-easier-to-clear-messages/5727/7