Improve autocomplete for "lastactivity" command.

[mdosch]

/lastactivity <tab> cycles through the setting on|off instead of offering you your contact JIDs.
It should be changed to require /lastactivity set<tab> to cycle through the setting on|off and cycle through contacts for /lastactivity <tab>.

Also it should be made more clear in the docs that /lastactivity on a server reports the servers uptime.

Environment

Master on Debian Bullseye (Testing)

profanity -v
Profanity, version 0.9.5dev.master.5a9f76c1
Copyright (C) 2012 - 2019 James Booth <boothj5web@gmail.com>.
Copyright (C) 2019 - 2020 Michael Vetter <jubalh@iodoru.org>.
License GPLv3+: GNU GPL version 3 or later <https://www.gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Build information:
XMPP library: libstrophe
Desktop notification support: Enabled
OTR support: Enabled (libotr 4.1.1)
PGP support: Enabled (libgpgme 1.13.1-unknown)
OMEMO support: Enabled
C plugins: Enabled
Python plugins: Enabled (3.8.4)
GTK icons: Enabled

Profanity 0.9.5 from backports on Debian Buster (Stable)

profanity -v
Profanity, version 0.9.5
Copyright (C) 2012 - 2019 James Booth <boothj5web@gmail.com>.
Copyright (C) 2019 - 2020 Michael Vetter <jubalh@iodoru.org>.
License GPLv3+: GNU GPL version 3 or later <https://www.gnu.org/licenses/gpl.html>

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Build information:
XMPP library: libstrophe
Desktop notification support: Disabled
OTR support: Enabled (libotr 4.1.1)
PGP support: Enabled (libgpgme 1.12.0)
OMEMO support: Enabled
C plugins: Enabled
Python plugins: Disabled
GTK icons: Disabled

[jubalh]

Also it should be made more clear in the docs that /lastactivity on a server reports the servers uptime.

Was already done in 7319f9e after you mentioned it in the MUC.