Consider factoring out all references to www.justfix.nyc as a global constant

[sraby]

"Since it's being repeated a number of times and we might want to eventually use it elsewhere on the site, it might be useful to factor this out into a JUSTFIX_WEBSITE_URL constant or something. Up to you tho, it's also easy enough to just do a find/replace later." - from https://github.com/JustFixNYC/tenants2/pull/1242#discussion_r412923621

[sraby]

Main question: do we want to do this across the Tenant Platform or just for NoRent?