inbound_email: add function to extract the plaintext from a mail
This function uses the html part if no text part is available.
Depends on !1064 (closed) Related to T3999
Test Plan
tests added
Migrated from D7503 (view on Phabricator)
This function uses the html part if no text part is available.
Depends on !1064 (closed) Related to T3999
tests added
Migrated from D7503 (view on Phabricator)