Fix hardcoded search method in header template

Should use GET if user has configured "GET only" in their config

Closes #446

app/templates/header.html

@@ -1,7 +1,9 @@
 {% if mobile %}
     <header>
         <div class="bz1lBb header-div">
-            <form class="search-form Pg70bf" id="search-form" method="POST">
+            <form class="search-form Pg70bf"
+                id="search-form"
+                method="{{ 'GET' if config.get_only else 'POST' }}">
                 <a class="logo-link mobile-logo" href="/">
                     <div id="mobile-header-logo">
                         {{ logo|safe }}
@@ -40,7 +42,10 @@
             </a>
         </div>
         <div class="search-div">
-            <form id="search-form" class="search-form" id="sf" method="POST">
+            <form id="search-form"
+                class="search-form"
+                id="sf"
+                method="{{ 'GET' if config.get_only else 'POST' }}">
                 <div class="autocomplete header-autocomplete">
                     <div style="width: 100%; display: flex">
                         <input