fix: Support description markdown and longer length (#385)

Fixes https://github.com/webrecorder/replayweb.page/issues/381

## Changes

- Renders description as markdown
- Shows "Collection name" and "About this collection" labels when
description is present
- Fixes overflow issue where bottom few pixels of embed is cut off
- Hides description in mobile view

Co-authored-by: emma <hi@emma.cafe>

src/appmain.ts

@@ -179,7 +179,8 @@ export class ReplayWebApp extends LitElement {
         flex-direction: column;
       }
       wr-item {
-        height: 100%;
+        flex: 1 1 auto;
+        overflow: hidden;
       }
       .navbar {
         padding: 0 0.5em;