Add tooltip to Activty Sync button

I couldn't figure out what the button does without clicking on it,
while for I could hover all the Strava-native buttons next to it and
figure out what they do.

templates/sync-button.html

@@ -1,4 +1,4 @@
-<div class="btn sauce-sync-button">
+<div class="btn sauce-sync-button" title="Sauce Activity Sync">
     <div class="sauce-sync-icon">
         {{=icon sync-alt-regular=}}
         {{=icon check-solid=}}