Add an advertising screen

[davidfischer]

This adds an advertiser directed page to the site where prospective advertisers can learn about advertising on RTD and ask for more information about advertising opportunities.

The advertising screen lives at /sustainability/advertising/ and when the form is submitted, the user will be redirected to /sustainability/advertising-submitted/ after an email is sent to rev@. This pull request does not update the footer to link to this page. If this is accepted, there are a number of places to update and that can come quickly in a second PR.

Some folders and files in this PR are named "advertisewithus" as opposed to "advertise" or "advertising" because some zealous ad blockers will block things named "advertise".

[ericholscher]

This looks great to me. We can do some basic iteration on it, but happy to ship this and start sending some traffic to it :)

[agjohnson]

All looks fine here. Again, we'll worry about organizing and reworking css later.

Any reason this was added to this repo? I thought we discussed moving things out to another repo? I'd prefer to trim down extraneous content we put into the .org codebase.

[ericholscher]

This got merged in the new donate repo (private, for now)