Improve signup and signout

- Fixes an issue with making a GET request to `/profile/sign_up` with no params (shouldn't accept GET anyway)
- Extracts sign out to a new `SessionController`
- Rename `UserController#signup` to `UserController#create`

This is helping towards work on https://github.com/mysociety/derechoapreguntar-theme/issues/14

**TODO:**

- [ ] Add changelog notes for renaming signup
- [ ] Remove `UserController#signup`
- [ ] Change `@user_signup` to `@user`

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