navbar: Switch make_tab_bar conditionals to using filter/sub values.
navbar: Switch make_tab_bar conditionals to using filter/sub values.

Previously, this function relied on the return value of
`filter.get_icon()` which made it brittle.
Directly using the properties of the filter and sub object makes this
more explicit about the intentions and robust.

punchagan/zulip,kou/zulip,andersk/zulip,shubhamdhama/zulip,shubhamdhama/zulip,hackerkid/zulip,rht/zulip,hackerkid/zulip,hackerkid/zulip,showell/zulip,andersk/zulip,timabbott/zulip,kou/zulip,showell/zulip,brainwane/zulip,zulip/zulip,timabbott/zulip,kou/zulip,rht/zulip,showell/zulip,eeshangarg/zulip,timabbott/zulip,synicalsyntax/zulip,zulip/zulip,hackerkid/zulip,hackerkid/zulip,synicalsyntax/zulip,timabbott/zulip,eeshangarg/zulip,shubhamdhama/zulip,andersk/zulip,rht/zulip,rht/zulip,brainwane/zulip,brainwane/zulip,zulip/zulip,kou/zulip,shubhamdhama/zulip,zulip/zulip,timabbott/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,synicalsyntax/zulip,brainwane/zulip,andersk/zulip,andersk/zulip,shubhamdhama/zulip,rht/zulip,showell/zulip,zulip/zulip,shubhamdhama/zulip,hackerkid/zulip,zulip/zulip,timabbott/zulip,kou/zulip,andersk/zulip,synicalsyntax/zulip,eeshangarg/zulip,showell/zulip,timabbott/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,eeshangarg/zulip,andersk/zulip,brainwane/zulip,rht/zulip,showell/zulip,synicalsyntax/zulip,eeshangarg/zulip,kou/zulip,synicalsyntax/zulip,synicalsyntax/zulip,punchagan/zulip,showell/zulip,kou/zulip,brainwane/zulip,brainwane/zulip,shubhamdhama/zulip,hackerkid/zulip,punchagan/zulip,punchagan/zulip,rht/zulip