Current Position Title Fixes

Change how positions behave to promote certain positions over others on member pages (eg the fact a person is Prime Minister over the fact they are a MP).
Firstly, it would be preferable to scrap any usage of this information that we can now better get from using more of data.parliament. From a quick look at a couple of random people, it looks like data.parliament covers everything that was in `chgpages/selctee`, and presumably also govposts, offoppose, libdem, etc. The only difference I found from random looking was some Plaid Cymru oddities that could be corrected at Parliament's side. So they could all be dropped apart from `privsec`.

2004-05-28 and 2004-05-13 are the first dates for data scraped from the old selctee/privsec pages on parliament.uk (you can see this in `cmpages/chgpages`' `privsec0001_2004-06-08.html` and `selctee0001_2004-06-08.html`). So if the date is those dates for those two things, you don't want to display it because it's not a known start date, it could have been before that date. 2005-11-10 is because the PPS changes did not all happen on that date but the website did not update until that date so it outputs "before" in either from/to date in that case. 2009-01-16 is the last date before the page disappeared off parliament.uk entirely so that displays that fact that after then we don't know.

@mhl "e.g. if the source is chgpages/privsec and to_date is 2005-11-10 it seems that this can output just the string: `to before` ?" - in full, it should output "PPS (Some Other MP) [1 Jan 2004] to before 10 Nov 2005" or something similar; ie they stopped at some point before that date, and we don't know when precisely.