use anchor element for social links when in game

[riknoll]

fixes https://github.com/microsoft/pxt-minecraft/issues/2668

the in-game webview on macos doesn't support any of the window open APIs that the browser provides.<a> elements work though, so this PR replaces the social buttons with <a> elements.

normally i would just replace these for all platforms since <a> elements are usually better for accessibility, but the only way to open a pop up window is with js and i think that's the expected behavior for social buttons. doesn't matter for the in-game experience given that those all open in the browser outside the game anyhow.

[abchatra]

@riknoll build failing

[riknoll]

@abchatra i'll fix it, but leave this one out of the release anyhow. i'd like more people to test it first so let's save it for the hotfix

[abchatra]

Sounds good.