LibDem scraper: with a URL as an argument, just scrape that person's page

This is useful for debugging: if you don't supply any command line
argument then all people are fetched, but if there's a URL as the first
argument, just that person is scraped and parsed.
LibDem scraper: clean up erratically formed Twitter accounts

Fixes https://github.com/mysociety/yournextmp-popit/issues/100
Add newly scraped Conservative candidates
Add newly scraped Labour data
Don't remove cropped image files on each run

This is important since some of these have been done manually, as
corrections to the automatic cropping.
Don't scrape the generic placeholder image for LibDems
Add some fixed LibDem Twitter usernames

These result from @tfgg's earlier fix to the scraper in
81dc95d7b539d0c985
Add new data from a run of the LibDem scraper
Add a function for tidying up erratically formed URLs
Tidy up URLs found via the LibDem scraper
Update broken URLs scraped from the LibDem official pages
Tidy up URLs found from the Labour scraper
Tidy homepage and Facebook URLs for Conservative candidates
Replace Conservative homepage URLs with tidied versions