Change class name "searchButton" to "kiwixButton"

This is done to retain the button design in more button designs (ex: tags)

README.md

@@ -198,7 +198,7 @@ To use JS provided by kiwix-serve you can use the following template to start wi
     ```
         <form id='kiwixSearchForm'>
         <input type="text" name="q" placeholder="Search" id="searchFilter" class='kiwixSearch filter'>
-        <input type="submit" class="searchButton" value="Search"/>
+        <input type="submit" class="kiwixButton" value="Search"/>
         </form>
     ```
 

static/skin/index.css

@@ -113,7 +113,7 @@ body {
     background-size: 15px;
 }
 
-.searchButton {
+.kiwixButton {
     margin: 0 17px;
     height: 35px;
     width: 100px;
@@ -123,7 +123,7 @@ body {
     border: solid 1px #b5b2b2;
 }
 
-.searchButton:hover {
+.kiwixButton:hover {
     background-color: #858585;
 }
 
@@ -434,7 +434,7 @@ body {
         margin-top: 11px;
     }
     
-    .searchButton {
+    .kiwixButton {
         margin: 19px 0;
         width: 229px;
     }

static/templates/index.html

@@ -58,7 +58,7 @@
       </div>
       <form id='kiwixSearchForm' class='kiwixNav__SearchForm'>
         <input type="text" name="q" placeholder="Search" id="searchFilter" class='kiwixSearch filter'>
-        <input type="submit" class="searchButton" value="Search"/>
+        <input type="submit" class="kiwixButton" value="Search"/>
       </form>
     </div>
     <div class="kiwixHomeBody">