Notifications: show attached to `User`, `Project` and `Organization`

[humitos]

Render notifications attached to these objects. This is the first pass of this work and there are some things we need to make decisions and improve.

• Where (what pages) these notifications should render?
• How "global" they should be considered?

Note we are rendering these notifications in the template for now, but in the future they will be rendered using the APIv3: #259

Screenshots

User

Project

Organization

Closes #260

[humitos]

I'm showing all the notifications attached to Project and Organization. However, the endpoint we are creating will only show these notifications to admins/owners. @agjohnson should I do the same here by returning a more specific queryset via the get_context() in the view?

[humitos]

I would put the project/organization notifications at the top of the content page, above the project/organization header.

Done in 84073ef

It looks like this now: