Partially fixed earlier in #467

fix #407

fix #410

now that we have client.destroyed again, we don't need to test the
"0 sessions && brb disabled" condition in both AddSession() and
destroy()

fix #527

Use []uint32 in bitset instead of []uint64, because it's harder to guarantee
64-bit alignment of []uint64 than I had realized:

https://go101.org/article/memory-layout.html

check restricted nicknames against skeletons, more pedantically

fix #521

Fixes #519

two more fixes

Quit messages aren't supposed to clobber each other under any
circumstances, but actually only the default "Connection closed"
message was non-clobbering, all others would clobber.

implement draft/resume-0.4

fix #511

fix #514

Also ensure that people can remove their own persistent modes via CS AMODE

Fixes #505

Setting `closed` on the socket (which can mean either "the socket is broken"
or "we should close the socket at the next opportunity") was racing against
the final write. Even if socket.closed is true, we should attempt to send
buffered message data to the socket, before we send the `finalData` and
actually call `Close`.

support znc.in/playback

fix #404

New Crowdin translations

make strict enforcement the default