Move current and former offices held code to the Member/Office classes.
Add full list of currently held offices to member pages.
Alter behaviour of current and former title positions to be more robust.
Modify current positions to only override the default if positions are
flagged as priority.
Rework parts of office listing to clarify the intent.
Better document the Office class, and reformat pretty_dates to be more readable.
Add explanation of the significance of dates, and a todo to update.
Change behaviour of "details" link on MP page policies.

Ensures the button works on touch devices.
New policy position on shares for rights

Closes #573
New policy position on local government powers

Closes #574
New policy position on secret courts

Closes #575
New policy position on communications data retention

Closes #621
New policy position on a register of lobbyists

Closes #622
New policy position on fees for lawyers in no-win no-fee cases

Closes #623
New policy position on letting agents' fees

Closes #624
New policy position on support for policies in the Coalition Agreement

Closes #626
Add paragraph to person pages explaining where we source vote data.

Closes #504
Update build status images and add installability badge for consistency.
Add mailing address to automated alert mailings.

Closes #640

As per discussion on #640, add our physical mailing address to automated
outgoing alert emails (ie those which just happen, and are not explicitly
initiated by a user).
Don't log anything with 'both' in the useragent.