[DEV-1842] Create sitemap for home/users/tags pages; prevent indexing of user profile pages

@geekofalltrades please review and +1.  AFAICT this PR can be merged without causing the /ask/* pages to be indexed; that won't happen until the corresponding PR in the web repo is merged (or someone submits /ask/sitemap.xml to Google, or Google discovers these pages organically).

https://roverdotcom.atlassian.net/browse/DEV-1842