presence: Fix confusing "active" label for "not unavailable".
presence: Fix confusing "active" label for "not unavailable".

The Help Center article talks about these using similar terms, which
may need further work, but it seems clear that undoing "set
unavailable" should be "set available", not "set active".

zulip/zulip,zulip/zulip,zulip/zulip,rht/zulip,eeshangarg/zulip,rht/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,punchagan/zulip,hackerkid/zulip,eeshangarg/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,andersk/zulip,punchagan/zulip,zulip/zulip,kou/zulip,rht/zulip,hackerkid/zulip,hackerkid/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,hackerkid/zulip,andersk/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,punchagan/zulip,punchagan/zulip,hackerkid/zulip,zulip/zulip,andersk/zulip,kou/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,kou/zulip,andersk/zulip,rht/zulip,zulip/zulip,punchagan/zulip