design: Make non-editable text-box more distinguishable.
design: Make non-editable text-box more distinguishable.

Earlier, the non-editable text-boxes(on clicking view source/edit
topic) were not so apparent due to absence of `disabled` attribute.
Adding the `disabled` attribute makes them consistent with the approach
for non-editable text-boxes and text-areas in organization settings
(for non-admins).

Fixes: #14375

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