Add share button

[nel-hei]

I have added a Share button that appears with and below the ‘Back to top’ button when scrolling to the main section of the website. Clicking on it displays a message that a share link has been copied.

Closes #21.

[v-ji]

Looks good so far! In terms of UX, let’s introduce an intermediary step between the share button and the link copying. This should be a floating dialogue next to the buttons which contains a small explanatory text (that the user can copy and share the link to this exact play). Below, the resulting URL should be visible (but truncated) along with a “copy” button.

With this increased scope, the component can be moved to its own file (in src/lib/components).

When initialising state from a shared URL, please trigger a scroll to the <main> element (already bound as mainElement). We may add onto this with a small notification or similar.

[nel-hei]

I've just noticed that if you manually change a single digit of the sequence by scrolling horizontally, the share link doesn't update. Still need to fix this.

[v-ji]

Hi Nele, thank you for your changes! Things are looking good and functional so far, just a few changes and comments:

I have removed the ?share=true URL parameter, as it is not needed to keep track of whether the user has arrived via a shared URL – any time sequence state is initialised from the URL hash, we know it’s from a shared URL.

Also, i fixed a race condition in the feature that scrolls to the main element when arriving from a shared URL: We first have to wait for the components to mount and the element to become available.

It would be great if you could address the individual comments before we can move into the specifics and also involve Mark for design/UX :)