navbar: Extract `reset_searchbox_text` in tab_bar.
navbar: Extract `reset_searchbox_text` in tab_bar.

The navbar currently fails at managing the searchbox text state in
cases where, eg, the user performs navigation by browser history.
This is a prep commit that will help resolve the bug.

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