added url_for function for use in Jinja2 templates

[kevinchai]

Added the app.url_for function for use in Jinja2 templates as requested in #87.

Useful for referencing static files and endpoints in html templates. e.g.

• {{ url_for('static', filename='styles.css') }}
• {{ url_for('dashboard') }}

url_for is available within Jinja2 templates for Flask by default as part of the standard context (https://flask.palletsprojects.com/en/2.2.x/templating/). There may be other useful variables / functions to also add by default. e.g. session for logged in user details.

[ahopkins]

This is a great addition! Thanks. Can you add a quick test also?

[kevinchai]

No worries. Just ran the tests with tox and fixed the identified errors with the second commit. Re-ran tests and they passed.