Add hotkey actions for page up/down in compose box.
Add hotkey actions for page up/down in compose box.

The hotkey actions make the caret position go to the top of the compose
box or bottom respectively, while the compose box is open.

Fixes #1910.

jainayush975/zulip,punchagan/zulip,synicalsyntax/zulip,zulip/zulip,rht/zulip,PhilSk/zulip,JPJPJPOPOP/zulip,rishig/zulip,PhilSk/zulip,jainayush975/zulip,Galexrt/zulip,brainwane/zulip,amanharitsh123/zulip,hackerkid/zulip,rht/zulip,shubhamdhama/zulip,JPJPJPOPOP/zulip,JPJPJPOPOP/zulip,showell/zulip,SmartPeople/zulip,brockwhittaker/zulip,susansls/zulip,dawran6/zulip,j831/zulip,souravbadami/zulip,blaze225/zulip,aakash-cr7/zulip,punchagan/zulip,dhcrzf/zulip,eeshangarg/zulip,jphilipsen05/zulip,dattatreya303/zulip,susansls/zulip,dhcrzf/zulip,hackerkid/zulip,vabs22/zulip,synicalsyntax/zulip,ryanbackman/zulip,jainayush975/zulip,jackrzhang/zulip,tommyip/zulip,PhilSk/zulip,shubhamdhama/zulip,christi3k/zulip,vaidap/zulip,timabbott/zulip,shubhamdhama/zulip,jrowan/zulip,andersk/zulip,aakash-cr7/zulip,jrowan/zulip,mahim97/zulip,brockwhittaker/zulip,dattatreya303/zulip,aakash-cr7/zulip,jphilipsen05/zulip,shubhamdhama/zulip,susansls/zulip,mahim97/zulip,jphilipsen05/zulip,christi3k/zulip,tommyip/zulip,andersk/zulip,ryanbackman/zulip,jackrzhang/zulip,j831/zulip,rht/zulip,blaze225/zulip,amanharitsh123/zulip,amanharitsh123/zulip,tommyip/zulip,hackerkid/zulip,hackerkid/zulip,blaze225/zulip,tommyip/zulip,jphilipsen05/zulip,rishig/zulip,mahim97/zulip,kou/zulip,mahim97/zulip,punchagan/zulip,jainayush975/zulip,kou/zulip,brainwane/zulip,susansls/zulip,ryanbackman/zulip,hackerkid/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,shubhamdhama/zulip,christi3k/zulip,brockwhittaker/zulip,dhcrzf/zulip,christi3k/zulip,shubhamdhama/zulip,vabs22/zulip,andersk/zulip,mahim97/zulip,aakash-cr7/zulip,timabbott/zulip,tommyip/zulip,rht/zulip,eeshangarg/zulip,dattatreya303/zulip,brainwane/zulip,dawran6/zulip,Galexrt/zulip,j831/zulip,zulip/zulip,synicalsyntax/zulip,hackerkid/zulip,zulip/zulip,christi3k/zulip,rishig/zulip,eeshangarg/zulip,vaidap/zulip,verma-varsha/zulip,christi3k/zulip,dhcrzf/zulip,Galexrt/zulip,dawran6/zulip,punchagan/zulip,blaze225/zulip,rht/zulip,jackrzhang/zulip,PhilSk/zulip,vaidap/zulip,ryanbackman/zulip,punchagan/zulip,Galexrt/zulip,kou/zulip,ryanbackman/zulip,dawran6/zulip,brockwhittaker/zulip,zulip/zulip,mahim97/zulip,jphilipsen05/zulip,jainayush975/zulip,souravbadami/zulip,synicalsyntax/zulip,amanharitsh123/zulip,Galexrt/zulip,tommyip/zulip,PhilSk/zulip,rishig/zulip,jrowan/zulip,brockwhittaker/zulip,brainwane/zulip,andersk/zulip,Galexrt/zulip,dhcrzf/zulip,showell/zulip,showell/zulip,andersk/zulip,timabbott/zulip,dattatreya303/zulip,jackrzhang/zulip,SmartPeople/zulip,JPJPJPOPOP/zulip,susansls/zulip,shubhamdhama/zulip,dhcrzf/zulip,SmartPeople/zulip,showell/zulip,j831/zulip,jainayush975/zulip,vaidap/zulip,zulip/zulip,brainwane/zulip,jackrzhang/zulip,rishig/zulip,vaidap/zulip,verma-varsha/zulip,rht/zulip,dattatreya303/zulip,amanharitsh123/zulip,verma-varsha/zulip,vaidap/zulip,verma-varsha/zulip,SmartPeople/zulip,jackrzhang/zulip,dhcrzf/zulip,jrowan/zulip,vabs22/zulip,showell/zulip,dawran6/zulip,j831/zulip,vabs22/zulip,timabbott/zulip,andersk/zulip,showell/zulip,vabs22/zulip,timabbott/zulip,dawran6/zulip,ryanbackman/zulip,zulip/zulip,synicalsyntax/zulip,Galexrt/zulip,dattatreya303/zulip,jrowan/zulip,rishig/zulip,eeshangarg/zulip,hackerkid/zulip,tommyip/zulip,eeshangarg/zulip,JPJPJPOPOP/zulip,jackrzhang/zulip,verma-varsha/zulip,showell/zulip,SmartPeople/zulip,zulip/zulip,souravbadami/zulip,amanharitsh123/zulip,SmartPeople/zulip,brockwhittaker/zulip,brainwane/zulip,j831/zulip,souravbadami/zulip,aakash-cr7/zulip,andersk/zulip,synicalsyntax/zulip,souravbadami/zulip,kou/zulip,blaze225/zulip,vabs22/zulip,timabbott/zulip,aakash-cr7/zulip,kou/zulip,susansls/zulip,kou/zulip,PhilSk/zulip,brainwane/zulip,JPJPJPOPOP/zulip,timabbott/zulip,synicalsyntax/zulip,verma-varsha/zulip,eeshangarg/zulip,rishig/zulip,jrowan/zulip,jphilipsen05/zulip,kou/zulip,punchagan/zulip,blaze225/zulip,souravbadami/zulip