Replace ex units with ems for input lengths

[querkmachine]

It was recently questioned why we were using ex units (a unit equivalent to the font's x-height) to define the width of inputs. Ultimately, the original reasoning appears to have been lost to time.

This PR replaces the use of ex units with em equivalents for Input lengths.

This makes sense as:

• em units are based on typeface width, rather than the height
• em and ex are both based on the font size, both affect the width of the input when the body font size is changed
• ex units are calculated differently by different browsers — IE 'cheats' and makes ex always equal half of em

Rather than make one-to-one replacements of ex for em, I rounded the em units up to the nearest .25. This prevents an excessive amount of decimal places in the resulting pixel measurements, but does mean that these inputs are up to a few pixels wider than previously.

Input length	old ex	new em	old px	new px
2	5.4	2.75	51.217	52.25
3	7.2	3.75	68.283	71.25
4	9.0	4.50	85.350	85.50
5	10.8	5.50	102.417	104.50
10	23.0	11.50	218.117	218.50
20	41.0	20.50	388.817	389.50
30	59.0	29.50	559.517	560.50

[vanitabarrett]

I'm pretty happy with this, looks good and I don't think the slight difference in width should have a big impact on anyone 👍

@owenatgov - you did the initial review. Do you want to re-review? Also happy for you to look at the failing Percy build and approve it if you'd like. First time I think we're actually doing that!

[owenatgov]

❌ 🙅🏻 Ⓜ️ 👌🏻