Allow <s> in markdown

doc/user/markdown.rst

@@ -145,7 +145,7 @@ to get a better plain text representation of your text. Note however, that for
 security reasons you can only use the following HTML elements::
 
     a, abbr, acronym, b, br, code, div, em, h1, h2,
-    h3, h4, h5, h6, hr, i, li, ol, p, pre, span, strong,
+    h3, h4, h5, h6, hr, i, li, ol, p, pre, s, span, strong,
     table, tbody, td, thead, tr, ul
 
 Additionally, only the following attributes are allowed on them::

src/pretix/base/templatetags/rich_text.py

@@ -65,6 +65,8 @@ ALLOWED_TAGS_SNIPPET = [
     'i',
     'strong',
     'span',
+    'strike',
+    's',
     # Update doc/user/markdown.rst if you change this!
 ]
 ALLOWED_TAGS = ALLOWED_TAGS_SNIPPET + [