[Welcome Page] Mark newly contributed walkthroughs as `new`

[JacksonKearl]

When an extension has newly contributed a walkthrough, it should be shown as New on the welcome page. It should remain New until it gets manually opened (not auto-opened via the install), or some number of days have passed (7?).\

Additionally, if an existing walkthrough adds new steps, it should be marked as having New Items, and should remain like this until it is opened.

[JacksonKearl]

\closedWith e03f9bd

[rzhao271]

I clicked on the new Luna Paint walkthrough, and then went back to the welcome view.
I noticed the new label was still there.
I repeated the process and the same thing occurred.
I resized the window to try and take a gif with gifcap, and then noticed the new label was gone by then.
I'm not sure how to reset the process.