Audit Icedove and TorBirdy as feed reader in Tails

This should be done against the questions raised in https://trac.torproject.org/projects/tor/wiki/torbirdy#IsitsafetosubscribetoRSSfeedswithThunderbirdandTorBirdy:

• Is automatic fetching disabled?
• Is HTML disabled?
• Is JavaScript disabled?
• Are proxy settings respected? (this is out of scope as far as Tails is concerned so not stricly required as part of this ticket)
• Are there other anonymity implications?

Related issues

• Related to #7626 (closed)
• Blocks #13234 (closed)

Original created by @sajolida on 11399 (Redmine)