change nav item logout link from get link to post button

[k-brahma]

As this is the first time tring to contribute OSS, sorry in advance if any of my procedures are not appropriate.

navbar logout link changed to post form.

Description

example/templates/_base.html fixed to:

<li class="nav-item">
<form action="{% url 'logout' %}" method="post" style="display: inline;">
    {% csrf_token %}
    <button type="submit" class="nav-link" style="background: none; border: none; padding-top: 8px; margin: 0; text-decoration: none; cursor: pointer;">
        {% trans "Logout" %}
    </button>
</form>
</li>

Motivation and Context

original link below causes 405 response. <a class="nav-link" href="{% url 'logout' %}">{% trans "Logout" %}</a>

How Has This Been Tested?

the test below actually ends with some errors but not differ from origin/master's result.

python .\example\manage.py test --settings=tests.settings   

Screenshots (if appropriate):

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.

[k-brahma]

Hi, I fixed it. Think that 'class="nav-link btn btn-link pt-2 m-0 text-decoration-none"' enough.

I visually confirmed that the nav-link works correctly in both expanded and collapsed states.

[k-brahma]

So I launched local sever and checked if the class propery works. Think it's enough.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (9159d1c) 95.52% compared to head (98a9e72) 95.52%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #691 +/- ## ======================================= Coverage 95.52% 95.52% ======================================= Files 78 78 Lines 3354 3354 Branches 377 377 ======================================= Hits 3204 3204 Misses 119 119 Partials 31 31 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.