Replace Seahorse and OpenPGP Applet with Kleopatra

Closes: #18875 (closed), #18845 (closed), #7435 (closed), #16104 (closed), #17223 (closed), #17371 (closed), #18484 (closed), #18747 (closed), #5316 (closed), #6568 (closed), #18723 (closed), #8309 (closed), #10992 (closed), #10991 (closed), #11394 (closed), #11188 (closed)

Note to reviewer: if !792 (comment 187440) was not completed yet while everything else is ready for merging, please turn that thread into a new tech writing issue and merge.