Move the gender field into BasePersonForm

This was accidentally only in NewPersonForm, so you couldn't edit it
after creating the person.

Fixes #47
Linkify the person ID in the action log
Fix an error where there's no party information for someone

In particular, there was an error thrown if there was no party
information for someone who was marked as definitely not standing in
2015.
Add some help about the suggested birth date format in its label

Ideally we'd make the date parsing more flexible, e.g. by using
a more liberal date parser, but just for the moment, due to time
constraints, at least suggest ISO 8601 date format in the label,
so people don't have to guess.

Workaround for #46
Thank Edmund for the 2010 YourNextMP data on the About page
Make sure each page has a helpful <title>

Fixes #52
Redirect to the same page after login or logout

Fixes #30