Add a User Preference to Receive only Plaintext Emails

This adds a user preference such that we only send plaintext emails to
this user. It also adds docs and tests.

For the user, this comes at the cost of not being in the same email
thread as the users who receive HTML emails. We decided to send out
plaintext emails on a one-by-one basis instead of making one thread (as
we do with HTML) as this would just create more confusion.

When admins enable inbound email for Gerrit, the aforementioned caveat
isn't a problem anymore as users would email Gerrit anyway instead of
using a notification email as messaging thread.

Change-Id: I3d0fc920c9d81e1a7b707ae5f3f4600a62b8a93b

Documentation/intro-user.txt

@@ -644,6 +644,27 @@ on comments that you write yourself.
 +
 Email notifications are disabled.
 
+- [[email-format]]`Email Format`:
++
+This setting controls the email format Gerrit sends. Note that this
+setting has no effect if the administrator has disabled HTML emails
+for the Gerrit instance.
++
+** `Plaintext Only`:
++
+Email notifications contain only plaintext content.
+Note that you will receive an individual email where you are the sole
+recipient while all other users that have HTML enabled will receive an
+email where they are all listed as recipients making it easier to start
+a discussion thread over email.
+The effect of this scenario is reduced if the administrator has enabled
+inbound email for this instance as replies are then parsed by Gerrit and
+Gerrit takes care of sending out new notification emails.
++
+** `HTML and Plaintext`:
++
+Email notifications contain both HTML and plaintext content.
+
 - [[default-base-for-merges]]`Default Base For Merges`:
 +
 This setting controls which base should be pre-selected in the