Make certificate descriptions sed safe

Make certificate descriptions sed safe

This escapes special characters used in POSIX sed to prevent mismatches.
e.g. `SYNO_Certficiate=*.example.com` would not match a description of
"*.example.com" and would look to match any number of double quotes (the
last character in the sed regex prior to certificate description),
followed by any single character, followed by "example", followed by any
character, followed by "com".

After this change, it will properly match `*.example.com` and not
`""zexamplefcom`.

Additionally we now store the certificate description as base64 encoded
to prevent issues with single quotes.

Tested on DSM 7.0-41222 (VDSM) and DSM 6.2.4-25556 (DS1515+).