Remove widget dependency, convert to blocks

[chillu]

There's a whole bunch of "widgets" which is our outdated way of doing blocks: https://github.com/silverstripe/silverstripe-blog/tree/master/src/Widgets

We don't have an equivalent replacement for this in elemental. Widgets are designed for a "sidebar" which should show up on every blog post. You can do this via https://github.com/dnadesign/silverstripe-elemental-virtual, but that's creating a lot of duplicate blocks, and needs to be configured again on every new blog post.

The concept of a configurable "sidebar" is kind of outdated. I think the baseline is giving devs the ability to add includes to their template for that functionality - since in most cases, they'll have to adjust the template anyway - take the silverstripe.org/blog for example, it has tags and categories.

In terms of widget types:

• Archive: Kind of useful for month based browsing on large blogs
• Categories and Tags: Should be a template addition? I don't think there's much value in a user-configurable limit and sort order
• Featured and Recent Posts: I think that's actually a more generic use case for a block module that just lists pages in a parent?
• Tag Cloud: I think we should drop support altogether, that can be a third party addition if anyone wants to recreate it...