[Account setup] Display "warning" message when using POP3

[cketti]

The app doesn't really support the "download and manage emails locally" model that is used with POP3. This will most likely remain true for a while. To avoid user frustration, we should display a warning message when users try to set up a POP account and suggest they use IMAP (or switch to a different app).

We could display a message below the "protocol" widget when "POP3" is selected.

[rohnsha0]

I'll do it! @cketti

[akashs056]

i am on it

[kewisch]

This issue has status: needs design, which normally means we'd like to wait for design to provide some input. Given this is "only" a message, if a contributor wants to work on this we'll ask the design team to review once the pull request is up.

The message should be encouraging people to use IMAP instead, not speaking negatively about POP3.

[Akshaygithubsd]

I'll do this @cketti