fix #520, render usernames containing custom emoji in welcome toast properly

What does this PR do? This PR fixes rendering of usernames containing custom emojis in the "Welcome Back, ${username}" toast message by wrapping the MkToast message in an Mfm tag. The plain argument for the m makes sure that for example "\n" won't get converted to a br element.

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assigned to @tess

added 2 commits

• ce197bc2 - add ability to conditionally wrap toast message in Mfm tag
• e7b87c02 - Pass renderMfm as true to wrap welcome back toast in Mfm

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mentioned in commit 8f3549b4