subdomain: Check for invalid characters before length.
subdomain: Check for invalid characters before length.

I think it makes more sense to first tell the user that
the character you are entering is invalid than telling
minimum length requirement is not satisfied.

Fixes #3058.

shubhamdhama/zulip,hackerkid/zulip,hackerkid/zulip,timabbott/zulip,eeshangarg/zulip,synicalsyntax/zulip,rishig/zulip,kou/zulip,punchagan/zulip,dhcrzf/zulip,synicalsyntax/zulip,kou/zulip,rht/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,rht/zulip,tommyip/zulip,andersk/zulip,synicalsyntax/zulip,timabbott/zulip,hackerkid/zulip,andersk/zulip,rishig/zulip,jackrzhang/zulip,zulip/zulip,shubhamdhama/zulip,dhcrzf/zulip,hackerkid/zulip,zulip/zulip,showell/zulip,shubhamdhama/zulip,punchagan/zulip,zulip/zulip,rht/zulip,brainwane/zulip,shubhamdhama/zulip,hackerkid/zulip,dhcrzf/zulip,punchagan/zulip,rht/zulip,eeshangarg/zulip,kou/zulip,kou/zulip,dhcrzf/zulip,brainwane/zulip,punchagan/zulip,dhcrzf/zulip,rishig/zulip,kou/zulip,showell/zulip,jackrzhang/zulip,timabbott/zulip,jackrzhang/zulip,zulip/zulip,dhcrzf/zulip,punchagan/zulip,tommyip/zulip,brainwane/zulip,brainwane/zulip,shubhamdhama/zulip,synicalsyntax/zulip,eeshangarg/zulip,rishig/zulip,brainwane/zulip,andersk/zulip,rishig/zulip,showell/zulip,eeshangarg/zulip,tommyip/zulip,kou/zulip,zulip/zulip,synicalsyntax/zulip,tommyip/zulip,timabbott/zulip,eeshangarg/zulip,showell/zulip,rht/zulip,showell/zulip,shubhamdhama/zulip,jackrzhang/zulip,dhcrzf/zulip,jackrzhang/zulip,jackrzhang/zulip,andersk/zulip,kou/zulip,punchagan/zulip,eeshangarg/zulip,punchagan/zulip,synicalsyntax/zulip,hackerkid/zulip,rishig/zulip,andersk/zulip,timabbott/zulip,jackrzhang/zulip,showell/zulip,timabbott/zulip,andersk/zulip,brainwane/zulip,tommyip/zulip,timabbott/zulip,rht/zulip,zulip/zulip,tommyip/zulip,showell/zulip,rishig/zulip,synicalsyntax/zulip,zulip/zulip,tommyip/zulip,brainwane/zulip,andersk/zulip,shubhamdhama/zulip