Construct mappings for businesslink_lrc

In order to have transition/bouncer take over responsibility for lrc.businesslink.gov.uk, we must
 - ensure there are appropriate mappings for transaction start and end pages (paths with ```/lrcOutbound``` and ```/lrcReturn```), for later use in transition 
 - handle the "commonly served" assets/attachments (where ```lrc.businesslink.gov.uk``` is considered the same as ```www.businesslink.gov.uk```)
 - shift the site to transition/sites

This pull request is for the first of those tasks, to provide mappings for the transaction start and end pages that are normally system-generated. This list was constructed using the 359 paths reported in [transition](https://transition.production.alphagov.co.uk/sites/businesslink_lrc/hits?page=1&period=all-time), presumably an exhaustive source of all 'important' mappings
Shouldn't this file be removed?