show the Mail-in-a-Box version in the control panel and a button to ping the MiaB website for the latest version

fixes #441

* Add a migration to delete any existing DKIM key so that existing machines get a fresh 2048-bit key. (Sadly we don't support key rotation so the change is immediate.)
* Because the DNS record for a 2048-bit key is so much longer, the way we read OpenDKIM's DNS record text file had to be modified to combine an arbitrary number of TXT record quoted ("...") strings.
* When writing out the TXT record value, the string must be split into quoted ("...") strings with a maximum length of 255 bytes each, per the DNS spec.
* Added a changelog entry.

Z-Push autoconfiguration fails due to URL case sensitivity 

Currently MiaB creates 1024 bit keys which is seen as a minimum standard
by several providers such as Google who already uses a 2048 bit key.
Increasing the keysize beyond 2048 is an issue as it often goes beyond
supported DNS record sizes.

Reject outgoing mail if MAIL FROM (envelope sender) does not match login name or is not an alias that directs mail (directly) to login name.

ownCloud breaks if download fails (Issue #449)

If the downloaded file doesn't pass hash verification, the script exits and leaves a broken system
Just make hash verification before moving owncloud directory

validate certificates using the cryptography python package as much as possible, shelling out to openssl just once instead of four times per certificate

* Use `cryptography` instead of parsing openssl's output.
* When checking if we can reuse the primary domain certificate or a www-parent-domain certificate for a domain, avoid shelling out to openssl entirely.

[JT added installing netcat-openbsd in system.sh]

add a new autoconfiguration option PRIMARY_HOSTNAME=auto to simply grab the hostname from reverse DNS

drawn from https://github.com/Toilal/mailinabox/commit/5b23a06a7410e4530a56fd6200a6c46c3c6ea9b6.

move more of start.sh into questions.sh to keep start.sh cleaner and encapsulate all of the variable setting in a single script

Based on https://github.com/Toilal/mailinabox/commit/5b23a06a7410e4530a56fd6200a6c46c3c6ea9b6.

fix wrong redirect for automatic www subdomain redirects

$ curl -I https://www.site.co.il/static/images/1.png?a=b | grep Location
Location: https://site.co.il?a=b
but should be something like 
Location: https://site.co.il/static/images/1.png?a=b

This was broken by the ability to have multiple TXT records in 9f1d633a.

fix loop redirecting

server is redirecting the request for this address in a way that will never complete

* Set ssl_stapling_verify to off per https://sslmate.com/blog/post/ocsp_stapling_in_apache_and_nginx ('on' has no security benefits).
* Set resolver to 127.0.0.1, instead of Google Public DNS, because we might as well use our local nameserver anyway.
* Remove the commented line which per the link above would never be necessary anyway.

OCSP seems to work just fine after these changes.