bootstrap4 tags page should have its own article/div HTML tag

[fvicent]

Hi there guys! I'm really comfortable using Nikola in a side project.

Requested Feature: Include an article or div HTML tag in the /bootstrap4/templates/tags.tmpl file.

Related Area: templates.

Do you want to contribute this yourself as a pull request?

• [x] Yes, I don’t have code ready yet

Does this feature affect backwards compatibility? If yes, in what way?

It might break custom CSS already applied to the tags pages in the bootstrap4 template and children.

Rationale and full description: By not having this HTML tag, it makes impossible applying custom CSS to the page content (i. e. the tags and categories lists) without affecting also the footer (such as centering the text). The base tags.tmpl already has this as <article class="tagindex">, so we could just mimic this approach.