[Glitch] Fix sidebar cut-off on small screens in admin UI

Port 7adde24a95 to glitch-soc

Signed-off-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/flavours/glitch/styles/admin.scss

@@ -384,7 +384,7 @@ $content-width: 840px;
           position: fixed;
           z-index: 10;
           width: 100%;
-          height: calc(100vh - 56px);
+          height: calc(100% - 56px);
           left: 0;
           bottom: 0;
           overflow-y: auto;