Standardize email plain-text templates and link generation

[ihsaan-ullah]

Description

• Add missing plain-text alternative for the account activation email, and send it via EmailMultiAlternatives instead of a plain-text-only EmailMessage.
• Rewrite 9 .txt templates that were literal copies of their .html counterparts (wrong , raw HTML tags) into proper plain text extending emails/base_email.txt.
• Replace hardcoded + Sites-framework links with across participation, forum, invite, and account-deletion-notice emails; fix links in the account-deletion admin notice that had no scheme at all.
• Add get_link_context() in utils/email.py as the single place that builds protocol/domain for email templates, and switch all senders to use it.

A checklist for email testing

Account activation

• Sign up a new user, confirm the activation email arrives
• View the email's plain-text version — no raw HTML tags visible
• View the HTML version — renders with Codabench branding/signature/footer
• Activation link uses the test server's actual domain and scheme (not production)
• Clicking the link activates the account successfully

Account deletion request

• Request account deletion, confirm the email arrives
• Plain-text version has no raw HTML tags
• Confirmation link uses the correct test-server domain/scheme
• Clicking the link completes the deletion flow

Account deletion — admin notice

• Delete a user who has organizations, owned competitions, participations, submissions, data, tasks, queues, and posts
• Admin notice email arrives to superuser/staff accounts
• Plain-text version has no raw HTML tags
• Every listed link (organizations, competitions, tasks) resolves correctly with the right domain/scheme

Account deletion — confirmed

• Complete a full deletion, confirm the "successfully removed" email arrives
• Plain-text version has no raw HTML tags

Competition participation — requested

• Apply to a competition as a participant (non-auto-approve, non-whitelisted)
• Participant receives "application sent" email with a working competition link
• Organizer(s) receive "user applied" email with a working competition link
• Both plain-text versions have no raw HTML tags

Competition participation — accepted

• Approve a pending participant (both manual approval and auto-approve/whitelist paths)
• Participant receives "accepted" email with a working competition link
• Organizer receives "accepted" email with a working competition link

Competition participation — denied

• Deny a pending participant
• Participant receives "denied" email with a working competition link
• Organizer receives "denied" email with a working competition link

Forum new-post notification

• Post a new reply in a competition forum thread where another user has forum notifications enabled
• Notification email arrives with a working thread link (correct domain/scheme)

Organization invite

• Invite a user to an organization
• Invite email arrives with a working accept/reject link (correct domain/scheme)
• Accepting the invite works end-to-end

General

• Confirm links point at the test server's domain, not production — verify DOMAIN_NAME/EXTERNAL_DOMAIN_NAME env vars on the test server are set correctly
• Spot-check that no email client (or "view plain text" toggle) shows literal <p>, <a>, <ul> etc. in any of the above

Checklist

• Code review by me
• Hand tested by me
• I'm proud of my work
• Code review by reviewer
• Hand tested by reviewer
• CircleCi tests are passing
• Ready to merge