On mobile screens when following a registration invite link, the page
would be too wide to interact with, extending well over the sides of the
phone.
This is because the minimum size of content is set to `auto` (by
default), and accord to the spec (https://www.w3.org/TR/css-flexbox-1/#specified-size-suggestion)
the minimum size of the element is the mimimum size of it's content. The
password strength element in the panel had a width of 500px, making the
entire component overflow the screen.
Changing the width to `flex-basis` instead, allows for the password
strength element to shrink if it overflows the screen.
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