extra blocks for templates allowing customization

[birddevelper]

Currently, adding a component to any part of the templates requires copying all the inherited template markup into the child template.

This PR refactors the content block into smaller, modular blocks, similar to how Django admin templates are structured. This allows developers to seamlessly insert components into a page without duplicating the entire template.

For example, to add a new button next to the existing top buttons:

{% extends "sqladmin/list.html" %}
{% block object_tools_items %}
{{ super() }}
<div class="ms-3 d-inline-block">
    <a href="some-link.html" class="btn btn-success">
        Some new button
    </a>
</div>
{% endblock %}

Result:

This PR also splits the head block into two separate blocks, extrahead and extrastyle , similar to Django admin templates.

[aminalaee]

I think this is great, can we just follow Flask-Admin to be aligned with other features?
https://flask-admin.readthedocs.io/en/v1.0.9/templates/#jinja2-flask-admin

[mmzeynalli]

@birddevelper update?

[birddevelper]

@mmzeynalli Will be updated soon. ;)

[mmzeynalli]

LGTM. But, I think it would be better to write tests for these, to make sure it is indeed in correct part of the HTML.

[mmzeynalli]

@birddevelper any updates with tests?

[birddevelper]

Sorry, I couldn't add the tests yet. Can you please help adding tests?