Fix password validation regex

[chriscarrollsmith]

• Make sure the following passwords pass HTML validation:

9g,X*88w[6"W
^94cPSf2^)z2^,&

• Write randomized tests to make sure validation works with many different combinations of numbers, upper/lower letters, and special characters
• Define password validation regex in one place and export to frontend and backend, programmatically escaping it for the frontend