Update builders.m.o location

[janbrasna]

One-line summary

Updates future.mozilla.org/builders link and relevant redirects to builders.mozilla.org

Significant changes and points to review

https://builders.mozilla.org is up now so this is good to go.

Issue / Bugzilla link

#15613

Testing

http://localhost:8000/builders/

[stephaniehobson]

@janbrasna needs a rebase, sorry!

[stevejalim]

@janbrasna The URL change will be handled initially by a redirect within future.m.o which will be pushed live at launch time. So feel free to remove the URL change from this PR if it makes things simpler

[janbrasna]

I understood the request the other way around; that until live the subdomain would redirect back to current future.m.o location.

I've separated out the ftl change to #15624 so this can sit around until needed.

[stevejalim]

The builders.m.o redirect in future.m.o is now live, FWIW

[stephaniehobson]

R+ 🏗️

[stephaniehobson]

@janbrasna Something went horribly wrong when I tried to fix a merge conflict 😫 Could you please rebase and force push?

[janbrasna]

@stephaniehobson NP, I had that ref already prepared & resolved, you were just quicker;) The merge did seem alright, but here's a clean rebase nonetheless, passing. ✅