push_notifications: Validate APNS token format in bouncer.
push_notifications: Validate APNS token format in bouncer.

This prevents a buggy old Zulip server from forwarding invalid-format
push notification tokens to the push notification bouncer service.

As part of this change, we switch the token from Text to str to match
the rest of the code path.

kou/zulip,mahim97/zulip,brainwane/zulip,verma-varsha/zulip,Galexrt/zulip,brainwane/zulip,jackrzhang/zulip,kou/zulip,Galexrt/zulip,amanharitsh123/zulip,jrowan/zulip,verma-varsha/zulip,jackrzhang/zulip,hackerkid/zulip,rishig/zulip,vabs22/zulip,tommyip/zulip,rht/zulip,showell/zulip,timabbott/zulip,shubhamdhama/zulip,Galexrt/zulip,jrowan/zulip,zulip/zulip,showell/zulip,hackerkid/zulip,synicalsyntax/zulip,shubhamdhama/zulip,kou/zulip,hackerkid/zulip,dhcrzf/zulip,synicalsyntax/zulip,andersk/zulip,vaidap/zulip,Galexrt/zulip,zulip/zulip,amanharitsh123/zulip,dhcrzf/zulip,zulip/zulip,Galexrt/zulip,dhcrzf/zulip,showell/zulip,andersk/zulip,mahim97/zulip,brockwhittaker/zulip,showell/zulip,hackerkid/zulip,showell/zulip,synicalsyntax/zulip,showell/zulip,zulip/zulip,verma-varsha/zulip,vaidap/zulip,eeshangarg/zulip,punchagan/zulip,zulip/zulip,brainwane/zulip,andersk/zulip,tommyip/zulip,showell/zulip,eeshangarg/zulip,mahim97/zulip,verma-varsha/zulip,timabbott/zulip,timabbott/zulip,mahim97/zulip,amanharitsh123/zulip,rishig/zulip,shubhamdhama/zulip,rishig/zulip,kou/zulip,dhcrzf/zulip,Galexrt/zulip,shubhamdhama/zulip,jrowan/zulip,eeshangarg/zulip,punchagan/zulip,synicalsyntax/zulip,brainwane/zulip,tommyip/zulip,punchagan/zulip,brainwane/zulip,brockwhittaker/zulip,vaidap/zulip,vaidap/zulip,jackrzhang/zulip,vaidap/zulip,hackerkid/zulip,rht/zulip,brockwhittaker/zulip,zulip/zulip,rht/zulip,andersk/zulip,jackrzhang/zulip,amanharitsh123/zulip,timabbott/zulip,punchagan/zulip,vabs22/zulip,hackerkid/zulip,brockwhittaker/zulip,brockwhittaker/zulip,dhcrzf/zulip,rht/zulip,timabbott/zulip,jackrzhang/zulip,synicalsyntax/zulip,kou/zulip,jrowan/zulip,shubhamdhama/zulip,vabs22/zulip,punchagan/zulip,verma-varsha/zulip,andersk/zulip,eeshangarg/zulip,eeshangarg/zulip,rishig/zulip,jrowan/zulip,brockwhittaker/zulip,shubhamdhama/zulip,tommyip/zulip,jackrzhang/zulip,tommyip/zulip,andersk/zulip,eeshangarg/zulip,kou/zulip,vabs22/zulip,shubhamdhama/zulip,timabbott/zulip,mahim97/zulip,verma-varsha/zulip,punchagan/zulip,timabbott/zulip,amanharitsh123/zulip,brainwane/zulip,rishig/zulip,vabs22/zulip,dhcrzf/zulip,mahim97/zulip,zulip/zulip,jackrzhang/zulip,kou/zulip,brainwane/zulip,rht/zulip,synicalsyntax/zulip,rishig/zulip,tommyip/zulip,hackerkid/zulip,amanharitsh123/zulip,punchagan/zulip,tommyip/zulip,vaidap/zulip,Galexrt/zulip,rht/zulip,rht/zulip,dhcrzf/zulip,rishig/zulip,eeshangarg/zulip,andersk/zulip,synicalsyntax/zulip,vabs22/zulip,jrowan/zulip