Make TextInput more mobile friendly in edit mode

[teberl]

Based on #9 and this comment from @sindresorhus i would purpose a small improvement to
.todo-list li.editing .edit.

The width attribute there is fixed in size by 506px. This fact makes it break on my phone in edit mode.

Since the rest of the design is already responsive and works on all sizes (i tested), i would suggest to make this width dynamic by using
width: calc(100% - 43px)
where the 43px reflect the margin-left we set for compensating the toggle element for completion.

Not sure about the browser support we have to guarantee, but this would work for most common browsers caniuse .