[simple] switch to the tablet layout at 79.75em

The previous value was 80em (1280px).
Some desktop screens have this resolution,
and tablet layout takes too much space in this configuration

This PR switch to the table layout for screen width strictly below 1280px.

Close https://github.com/searxng/searxng/issues/874

searx/static/themes/simple/src/less/definitions.less

@@ -244,7 +244,7 @@
 
 /// Device Size
 /// @desktop > @tablet
-@tablet: 80em;
+@tablet: 79.75em;  // see https://github.com/searxng/searxng/issues/874
 @phone: 50em;
 @small-phone: 35em;
 @ultra-small-phone: 20rem;