Post organisation message in own name

Company updates are always shown as the organisation. An admin who can post them cannot post the same update in their own name.

What we'll add:

• A choice, while creating a company update, between the organisation and the admin's own name and profile picture. The update stays on the company updates timeline either way.

• The organisation remains the default, including for updates already posted.

• The chosen name is what everyone sees on the timeline, on the message, and on notifications.