Add support for Gemini urls (#15013)

This PR updates the `valid_url` regex and sanitizer allowlist to provide
support for Gemini urls.

Closes #14991

app/lib/sanitize_config.rb

@@ -18,6 +18,7 @@ class Sanitize
       gopher
       xmpp
       magnet
+      gemini
     ).freeze
 
     CLASS_WHITELIST_TRANSFORMER = lambda do |env|

config/initializers/twitter_regex.rb

@@ -29,7 +29,7 @@ module Twitter
       (                                                                                     #   $1 total match
         (#{REGEXEN[:valid_url_preceding_chars]})                                            #   $2 Preceding character
         (                                                                                   #   $3 URL
-          ((?:https?|dat|dweb|ipfs|ipns|ssb|gopher):\/\/)?                                  #   $4 Protocol (optional)
+          ((?:https?|dat|dweb|ipfs|ipns|ssb|gopher|gemini):\/\/)?                           #   $4 Protocol (optional)
           (#{REGEXEN[:valid_domain]})                                                       #   $5 Domain(s)
           (?::(#{REGEXEN[:valid_port_number]}))?                                            #   $6 Port number (optional)
           (/#{REGEXEN[:valid_url_path]}*)?                                                  #   $7 URL Path and anchor