right sidebar: Improve icon for "Invite more users".
right sidebar: Improve icon for "Invite more users".

This replaces the `+` with a version that has a user head next to it.
rht/zulip,punchagan/zulip,zulip/zulip,kou/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,andersk/zulip,kou/zulip,rht/zulip,eeshangarg/zulip,punchagan/zulip,kou/zulip,punchagan/zulip,hackerkid/zulip,punchagan/zulip,eeshangarg/zulip,rht/zulip,zulip/zulip,andersk/zulip,zulip/zulip,andersk/zulip,andersk/zulip,kou/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,zulip/zulip,punchagan/zulip,andersk/zulip,eeshangarg/zulip,hackerkid/zulip,punchagan/zulip,kou/zulip,hackerkid/zulip,kou/zulip,hackerkid/zulip,rht/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip