Update Volunteer onboarding email with GitHub access

[Mariatta]

Accepted volunteers will be added to PyLadiessCon GitHub team.

• Adjust the welcome and onboarding email with GitHub information.

1. Admins/Team leads/ core teams will be added to @pyladies/global-conference-organizers team
2. Volunteers and team members will be added to @pyladies/pyladiescon-volunteers team

• Include a check in the email template to remind people about GitHub access.

For example,

{% if not profile.github_username %}
Your GitHub profile is currently missing. Please update it on the portal and let your team lead know after you've done that.
{% endif %}

• Show a banner when they logged in asking them to update their GitHub