Add option to disable seeking on transcodes. (Mitigates #548 & #723)

As per #548, #723, and tsquillario/Jamstash#131, the current method of
estimating `Content-Length` creates various problems.

However, if headers such as `Accept-Ranges` is omitted, clients will only
use the first connection, which is `Transfer-Encoding: chunked`, and no
`Content-Length` is necessary.

Doing this has the side effect that (at least on the web player) seeking
to a specific time is no longer possible, thus this was made an opt-in
option.

Signed-off-by: WillyPillow <wp@nerde.pw>

airsonic-main/src/main/webapp/WEB-INF/jsp/transcodingSettings.jsp

@@ -59,6 +59,11 @@
 
 
     <table style="white-space:nowrap" class="indent">
+        <tr>
+            <td>
+                <input type="checkbox" id="enableSeek" name="enableSeek" class="checkbox" ${model.enableSeek ? "checked" : ""}/>
+                <label for="enableSeek"><fmt:message key="transcodingsettings.enableseek"/></label>
+            </td>
         <tr>
             <td style="font-weight: bold;">
                 <fmt:message key="advancedsettings.downsamplecommand"/>
@@ -95,4 +100,4 @@
 <div style="width:60%">
     <fmt:message key="transcodingsettings.info"><fmt:param value="${model.transcodeDirectory}"/><fmt:param value="${model.brand}"/></fmt:message>
 </div>
-</body></html>
+</body></html>