Make Mark All As Read optional

[karlhorky]

I just noticed the new "Mark all as read" link that was added in this PR by @tjx666:

• https://github.com/tanmayrajani/notifications-preview-github/pull/152

It seems like a nice feature! Maybe even good enough to be default for most users 👍

But for me (and maybe also others), it makes me nervous that I will accidentally click on it and mark everything as read. I often keep notifications as a kind of ad-hoc todo list for myself.

Also, it takes up extra vertical space, which I would like to avoid:

So I added an option (default: link enabled), which allows users to disable the link if they like.

cc @fregante what do you think?

[karlhorky]

Apart from the approval by @tjx666, anything else I can do in this PR to make it ready?

[tjx666]

fine.

[karlhorky]

Thanks for the review, merge and publish of notifications-preview-github@24.8.19 🙌