Add plaintext quote marker to blockquote lines in one pass

Looking over the code again, I realized that there's no reason to extract blockquotes and then add them back later. I should have made this change in my last commit.

I deleted ComposerPageStateTest.resolve_nesting and ComposerPageStateTest.quote_lines because they refer to functions that no longer exist and are in any case redundant to ComposerWebViewTest.get_text_with_quote and ComposerWebViewTest.get_text_with_nested_quote.