Create a JustFix-wide content guide and enforce it

[toolness]

So there's a bunch of little copy-related things that we collectively have some confusion over, e.g.:

• Capitalization of potentially proper nouns: is it "Certified Mail" or "certified mail", "Landlord" or "landlord", "Letter of Complaint" or "letter of complaint", etc.
• Are we using title case or letter case for our titles? (I prefer letter case, FWIW.)
• Do we label things with the first person ("start my letter") or second person ("start your letter")? See is this my interface or yours for some guidance on this.
• Spelling/capitalization of "JustFix.nyc" (now I know that it should always be spelled this way, but it was communicated to me verbally and it'd be nice to have it written down somewhere).

This isn't only relevant to copy on the tenants app, of course, so it might be good to put it somewhere that folks working on other aspects of JustFix.nyc can easily access.

While part of me thinks we should go all-out and fork the 18F content guide, we could also just start a Markdown page somewhere and build on it.

[toolness]

We should also mention Tahnee's approach to capitalization described in https://github.com/JustFixNYC/tenants2/pull/1405#issuecomment-627415847.