[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>

static/style.css

@@ -453,3 +453,38 @@ button {
         color: #d8d3cb;
     }
 }
+
+/* find-feed */
+.search-result {
+  background-color: #f0f0f0;
+  border-radius: 5px;
+  padding: 15px;
+  display: flex;
+  position: relative;
+  text-align: left;
+}
+@media (prefers-color-scheme: dark) {
+    .search-result {
+        background-color: #202325;
+    }
+}
+.search-result h2 {
+  color: #288cfc;
+}
+
+.search-result a {
+  text-decoration: none;
+  color: #248afa;
+}
+.search-result .icon {
+  margin: 0 15px 0 0;
+}
+.search-result span {
+  margin-right: 10px;
+}
+.search-result .description {
+  font-size: 110%;
+  margin-right: 0 !important;
+  margin-top: 5px !important;
+}
+/* end find-feed */