Add referral tags to home & account product links

[janbrasna]

One-line summary

Adds most prominent missing utm_params to products / own external links.

Significant changes and points to review

• This is just the low-hanging fruit for add UTM codes for links to monitor.mozilla.org #14432 with most impact, will look around some more for less prominent locations eventually, too.
• Also removes some passé branding switches, leaving that to ftl fallbacks now.
• Removes some /account ftl conditional logic that mostly doesn't work, when at it.

Issue / Bugzilla link

#14432

Testing

http://localhost:8000/en-GB/
http://localhost:8000/en-GB/account/

[janbrasna]

Notes for reviewer:

[janbrasna]

Usually the utm_tags use _entrypoint values, but here the location is no longer valid (/firefox/accounts moved to /accounts) so I'm using params independent on it.

At the same time I'm not updating the _entrypoint value so the data collected stays consistent even when it doesn't reflect the right location today.

[alexgibson]

r+ thanks!