Support for vnd.dovecot.pgp-encrypt

[kaivol]

Added (rudimentary) support for the vnd.dovecot.pgp-encrypt sieve filter.

[kaivol]

Thank you for your response!

Please have a look at the last changes as I am not really familiar with this project and its best practices. Unfortunately this also applies to the vnd.dovecot.pgp-encrypt sieve extension, and I could not find much info about it. Primarily I made sure that my implementation is compatible with the with the way mailbox.org uses the extension.

I also updated the author in the license headers and removed the i18n strings in other languages than en-us. Originally, I added those because the application would complain about missing translations after adding the English translations only. Did I use the build system wrong or if not, what is the recommended workflow when introducing new translations?

[yoshimo]

Where would i find the dovecot side of this PR and its documentation?

[kaivol]

Unfortunately I could not find documentation or source code for the extension. I asked my provider (mailbox.org) about it, but have not yet received an answer.

[kaivol]

According to mailbox.org, no public documentation for this plugin is available. It was conceived by them and developed by Dovecot (initially exclusively for mailbox.org).

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[thsmi]

Sorry completely forgot about this pull request. Will look at it in the next days.