Fix #1907 – Obfuscate contact email addresses in public HTML (#5477)

* Include nix development enviornment

* Obfuscate contact email addresses in shop HTML and deanonymize via JavaScript

This change addresses #1907: "hide contact e-mail address in source code
of a shop".

- Contact email addresses rendered in public-facing templates are now
obfuscated in the HTML source (e.g., replacing "@" with "[at]" and "."
with "[dot]").
- A new JavaScript file is included in the relevant templates to
automatically rewrite and restore the email address for users after the
page loads.
- This approach helps protect email addresses from basic harvesting bots
and reduces spam, while keeping them accessible and user-friendly for
human visitors.
- The obfuscation and deanonymization logic is only applied to web
templates, not to emails sent via pretix.

This implementation follows the recommendations discussed in #1907,
using a standardized, maintainable approach that’s compatible with
pretix's asset pipeline and template structure.

* Undo nix development environment for merge into main

* convert complete mailto-link to HTML entities

* remove gitignore noise

* Update .gitignore

* fix gitignore noise

* Update .gitignore

---------

Co-authored-by: Richard Schreiber <schreiber@rami.io>

src/pretix/base/templatetags/anonymize_email.py

@@ -0,0 +1,13 @@
+from django import template
+from django.utils.html import mark_safe
+
+register = template.Library()
+
+
+@register.filter("anon_email")
+def anon_email(value):
+    """Replaces @ with [at] and . with [dot] for anonymization."""
+    if not isinstance(value, str):
+        return value
+    value = value.replace("@", "[at]").replace(".", "[dot]")
+    return mark_safe(''.join(['&#{0};'.format(ord(char)) for char in value]))