Change reply behavior

Sometimes users seem to be surprised about the way K-9 Mail selects recipients when the "reply" action is used. I'm proposing to add more options with names that hopefully make it clearer which addresses will be selected as recipients.

Proposed behavior described via examples:

From: sender@example.org
To: you@example.org

Displayed actions (in order):

• Reply (sender@example.org)

---

From: sender@example.org
To: unknown@example.org

Displayed actions (in order):

• Reply (sender@example.org)
• Reply to all (sender@example.org, unknown@example.org)

---

From: sender@example.org
To: you@example.org
Reply-To: other@example.org

Displayed actions (in order):

• Reply (other@example.org)
• Reply to all (sender@example.org, other@example.org)

---

From: sender@example.org
To: unknown@example.org
Reply-To: other@example.org

Displayed actions (in order):

• Reply (other@example.org)
• Reply to all (sender@example.org, other@example.org, unknown@example.org)

---

From: sender@example.org
To: you@example.org
List-Post: <mailto:list@example.org>

Displayed actions (in order):

• Reply to list (list@example.org, sender@example.org)
• Reply (sender@example.org)

---

From: sender@example.org
To: you@example.org
Reply-To: list@example.org
List-Post: <mailto:list@example.org>

Displayed actions (in order):

• Reply to list (list@example.org, sender@example.org)
• Reply (sender@example.org)

---

From: sender@example.org
To: you@example.org
Reply-To: other@example.org
List-Post: <mailto:list@example.org>

Displayed actions (in order):

• Reply to list (list@example.org, other@example.org)
• Reply (other@example.org)
• Reply to all (sender@example.org, other@example.org, list@example.org)

---

From: sender@example.org
To: list@example.org
Reply-To: list@example.org
List-Post: <mailto:list@example.org>

Displayed actions (in order):

• Reply to list (list@example.org, sender@example.org)
• Reply (sender@example.org)

---

From: sender@example.org
To: list@example.org
Reply-To: other@example.org
List-Post: <mailto:list@example.org>

Displayed actions (in order):

• Reply to list (list@example.org, other@example.org)
• Reply (other@example.org)
• Reply to all (sender@example.org, other@example.org, list@example.org)

---

From: sender@example.org
To: unknown@example.org
Reply-To: list@example.org
List-Post: <mailto:list@example.org>

Displayed actions (in order):

• Reply to list (list@example.org, sender@example.org)
• Reply (sender@example.org)
• Reply to all (sender@example.org, list@example.org, unknown@example.org)

---

From: sender@example.org
To: unknown@example.org
Reply-To: other@example.org
List-Post: <mailto:list@example.org>

Displayed actions (in order):

• Reply to list (list@example.org, other@example.org)
• Reply (other@example.org)
• Reply to all (sender@example.org, other@example.org, list@example.org, unknown@example.org)

---

Feedback is very welcome.

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Update 1: Incorporated feedback from https://github.com/k9mail/k-9/issues/3738#issuecomment-440997017
Update 2: See https://github.com/k9mail/k-9/issues/3738#issuecomment-441495208

Start by reviewing the proposed recipient examples and the feedback linked in the issue comments. Trace the existing reply-action behavior in the Android app, then verify that the displayed actions and recipient lists match the documented cases.