Add option to allow changing the font used to display emails

Expected behavior

When I receive a string of letters, for example a password reset, I expect to be able to distinguish the letter L form the letter i.

Actual behavior

I can not believe how often I still see this issue, it is 2018!
the current font k9 uses on my android phone shows the lower L and the capital i as the same vertical line. It is not possible to distinguish them visually. This is incredibly frustrating, because any computer user has know this issue for over a decade, and it still exists in various softwares

The solution is so simple: give preference to fonts with show a capital i as a capital H rotated 90 degrees.
Please consider selecting a different default font for K9, or somehow modifying the letter I on the current font.
I know the font is usually supplied by the operating system, and changing fonts can have a range of consequences, but I am sure we can work out a solution for this.

Environment

K-9 Mail version:
5.600

Android version:
7.0 3.18.31

Account type (IMAP, POP3, WebDAV/Exchange):
IMAP

The issue names no source file or test. Start by locating the Android email-rendering UI and its font configuration; done means the selected or default font clearly distinguishes ambiguous letters while email display remains usable.