Canada.ca template: Make language swap toggle dynamic based on current URL

[gordlin]

Description

On the Canada.ca template, clicking on the language swap toggle (Francais/English) always brings you to the other-language version of the page you initially loaded into, no matter what page you're currently on.

Steps to reproduce

1. Go to any product on the Canada.ca template
2. Swap languages. Since you're still on the same page, it works fine.
3. Change pages (e.g. get into the main editor page).
4. Swap languages. You're now sent to the other-language version of the page you initially loaded into (in step 1/2)!

Expected behavior

The link in the language swap toggle should be dynamic based on the current URL. It should send you to the current page in the other language.

Actual behavior

The toggle sends you to the initially-loaded page in the other language

Additional information

This is because the code for the toggles (in index-ca-en.html and index-ca.fr.html) hard-codes the links for the toggles on initial creation; whatever the links were at the beginning will be what they are throughout the session, until a page reload.

Related issues

Discussed in #530 (review)