dev: Migrate grid new

[philhanna]

Old behavior

• In menu.html, click on menu option "New Grid...". This calls javascript function do_grid_new() in crossword.js
• do_grid_new() makes the grid-new.html page visible
• grid-new.html prompts for an odd positive integer, validating it on submit with validateNewGridForm()
• validateNewGridForm gets the value: let n = document.forms["gn-form"]["n"].value; and checks:
  • That n is a number
  • That n is odd
  • That n is positive
• If grid is valid, calls {{ url_for('uigrid.grid_new') }}

New behavior

• do_grid_new function moved to grid.js
• do_grid_new uses input box with:
  • title: "New grid"
  • label: "<b>Grid size:</b> <em>(a single odd positive integer)</em>"
  • value: ""
  • action: "javascript:validateNewGridFormAndSubmit()"
  • method: blank
• New function validateNewGridFormAndSubmit is the same as its predecessor except that if the number passes, the uigrid.grid_new function is called.