[Core] New feature : User Interface to "Detect" Feed from an URL (#3436)

* [Core] New feature : User Interface to "Detect" Feed from an URL

Detect Action has been expanded to support returning a Feed in a JSON
format instead of a Redirect. Existing usage of the Detect action will
keep working as usual.

Frontpage template has now a section to display the Feed detection
result, and a button to start the Feed Detection.

A new JS file contains the necessary JS (Ajax and Event management) to
fill the Feed Detection section.

* Coding policy fixes

* [Core] New feature : User Interface to "Detect" Feed from an URL

- Switch from old school XMLHttpRequest to fetch
- Enhance UX of search results
- Revert to it's original content
- Switch to a new Action : FindfeedAction.php
- Switch to template literals instead of string concatenation
- FindFeed action could retrun multiple feeds
- Results are sent with an absolute URL
- Switch to Json::encode() helper function

* [Core] New feature : User Interface to "Detect" Feed from an URL

- Move specific JS code to rss-bridge.js
- Change HTML tag for the button to have a consistant style with th rest
  of the page

* [Core] New feature : User Interface to "Detect" Feed from an URL

- If no context is sent, assume there is only one unnamed context
- Find parameter name in global and currect context

* fix

* remove typo

---------

Co-authored-by: Dag <me@dvikan.no>

templates/frontpage.html.php

@@ -2,6 +2,7 @@
 <script>
     document.addEventListener('DOMContentLoaded', rssbridge_toggle_bridge);
     document.addEventListener('DOMContentLoaded', rssbridge_list_search);
+    document.addEventListener('DOMContentLoaded', rssbridge_feed_finder);
 </script>
 
 <section class="searchbar">
@@ -15,6 +16,14 @@
         onkeyup="rssbridge_list_search()"
         value=""
     >
+    <button
+        type="button"
+	    id="findfeed"
+        name="findfeed"
+    />Find Feed from URL</button>
+    <section id="findfeedresults">
+    </section>
+
 </section>
 
 <?= raw($bridges) ?>