Merges the design experiment at
https://meta.discourse.org/t/post-quote-copy-to-clipboard-button-feedback/285376
into core.
This adds a new button by default to the menu that pops up when text is
selected in a post.
The normal Quote button that is shown when selecting text within a post
will open the composer with the quote markdown prefilled.
This new "Copy Quote" button copies the quote markdown directly to the
user’s clipboard. This is useful for when you want to copy the quote
elsewhere – to another topic or a chat message for instance – without
having to manually copy from the opened composer, which then has to be
dismissed afterwards. An example of quote markdown:
```
[quote="someuser, post:7, topic:285376"]
In this moment, I am euphoric.
[/quote]
```
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