Add attribution to Tabler.io icons used in the new mailer designs

[ClearlyClaire]

This moves some files to a different directory, thus breaking their usage in mails, but only mails from the very last commit would use them.

[mjankowski]

Side note to this change -- is it correct at this point that all the mailers have been converted and are using the new layout, assets, etc? -- and if so, we can probably a) delete the old unused mailer-only assets, b) rename the mailer-new to just mailer or whatever once the old stuff is gone?

[renchap]

Side note to this change -- is it correct at this point that all the mailers have been converted and are using the new layout, assets, etc? -- and if so, we can probably a) delete the old unused mailer-only assets, b) rename the mailer-new to just mailer or whatever once the old stuff is gone?

Yes, I think they have all been converted.

But we probably want to keep the old assets, at least for some time, because they might have been referenced in old emails, and if people run clobber (or the docker image), it would make them 404.