Reduce PGP spam, minor bugfixes

[BjarniRunar]

This PR has 4 things:

1. Attempt to avoid sending confusing PGP keys to recipients who cannot make use of them
2. Make PGP keys and signatures less confusing by wrapping in friendly HTML
3. Stop adding Content-ID MIME headers
4. Fix a bug in the open-redirect security fixes (Prevent external redirects. Fixes #1582 #1695)

Of these, 3 and 4 are just bugfixes. 1 and 2 are the fun stuff, based on the discussion in #1693.

The output of 2 has been tested with mailpile, mutt, kmail and gpgol. The signatures worked fine in all cases, the keys worked well in all but gpgol (and the dev is looking into that).