Add privacy and tos links to email [MJ-365]

Use EmailMessage class rather than send_mail method to send email
Migrate all emails to html mail to allow for pretty-formatted links
Prevent click tracking for unsub, tos, and privacy links
Display privacy and tos links differently in emails [MJ-365]

Formatting of links changed
Links added to new table row in saved search emails
Merge branch 'master' into email-add-tos-and-privacy__MJ-365
Merge pull request #246 from DirectEmployers/email-add-tos-and-privacy__MJ-365

Add privacy and tos links to email [MJ-365]