feat(backends): redirect to another backend if one is unavailable

Only works with cookies for now. Support for numbered backends will be
added later since it requires some black magic.

src/invidious/routes/before_all.cr

@@ -48,6 +48,17 @@ module Invidious::Routes::BeforeAll
           env.response.cookies[CONFIG.server_id_cookie_name] = Invidious::User::Cookies.server_id(host, current_companion)
         end
 
+        companion_status = BackendInfo.get_status
+
+        if companion_status[current_companion] != 2
+          alive_companion = companion_status.index(2)
+          if alive_companion
+            env.set "companion_switched", true
+            current_companion = alive_companion
+            env.response.cookies[CONFIG.server_id_cookie_name] = Invidious::User::Cookies.server_id(host, current_companion)
+          end
+        end
+
         env.set "current_companion", current_companion
 
         if host.split(".").last == "i2p"

src/invidious/views/template.ecr

@@ -112,6 +112,7 @@
 					domain = env.get?("using_domain")
 					scheme = env.get("scheme")
 					status = BackendInfo.get_status
+					companion_switched = env.get?("companion_switched")
 				%>
 				<div class="h-box" style="margin-bottom: 10px;">
 					<b>Switch Backend:</b>
@@ -149,6 +150,12 @@
 				</div>
 				<% end %>
 
+				<% if companion_switched %>
+					<div class="h-box">
+						<p><%= translate(locale, "backend_unavailable") %></p>
+					</div>
+				<% end %>
+
 				<% if CONFIG.banner %>
 					<div class="h-box">
 						<h3><%= CONFIG.banner %></h3>