Preferences: Fix handling of modified source code URL(#4437)

Before this PR, setting the modified code repo URL through the preferences
page in Invidious was broken:

* the HTML input tag for this field had invalid type "input"
  (though browser falls back on text input)

* the URL was used to set the "checked" property and not as a plain value,
  which makes no sense for a text-based input (and resulted in a blank field)

* when the submitted field is empty, the retrieved value was an empty 'String'
  instead of 'nil', causing the "modified source code URL" to be an empty
  'href' link which just pointed to the current page

No associated open issue

src/invidious/routes/preferences.cr

@@ -214,7 +214,7 @@ module Invidious::Routes::PreferencesRoute
         statistics_enabled ||= "off"
         CONFIG.statistics_enabled = statistics_enabled == "on"
 
-        CONFIG.modified_source_code_url = env.params.body["modified_source_code_url"]?.try &.as(String)
+        CONFIG.modified_source_code_url = env.params.body["modified_source_code_url"]?.presence
 
         File.write("config/config.yml", CONFIG.to_yaml)
       end

src/invidious/views/user/preferences.ecr

@@ -310,7 +310,7 @@
 
                 <div class="pure-control-group">
                     <label for="modified_source_code_url"><%= translate(locale, "adminprefs_modified_source_code_url_label") %></label>
-                    <input name="modified_source_code_url" id="modified_source_code_url" type="input" <% if CONFIG.modified_source_code_url %>checked<% end %>>
+                    <input name="modified_source_code_url" id="modified_source_code_url" type="url" value="<%= CONFIG.modified_source_code_url %>">
                 </div>
             <% end %>