Fix libdem twitter accounts

Fixed Twitter accounts as scraped by libdem scraper, Issue mysociety/yournextmp-popit#100 .

Other cases of bad Twitter accounts were found by running

    grep -i twitter *

in the json/libdem directory and inspecting by eye. The new code seems to get all the accounts correct and *might* be more robust for future mistakes. Ultimately we're limited by the inconsistency of the libdem website.