Downloads: Avoid using word 'Bootstrap' and change header to 'Import Blockchain'

Translators may find this term confusing, especially since we use it in 2 different contexts (bootstrap node, blockchain boostrap). See https://translate.getmonero.org/translate/getmonero/monero-site/ru/?q=has%3Acomment&offset=1#comments

Let's change that header into something less confusing and less technical like "Import blockchain".