Accessibility fixes

Accessibility fixes in response to DAC review.

File ref MAS `mortgage calculatormobile report 191114.doc`.

Notable changes:

* Angular directive to autoselect text in text fields on focus.
* Angular directive to convert `a` tag to functionally identical `button` tag when JS is present. Basic use-case is that DAC prefers consistency between UI buttons (i.e. they should all be links, or all the buttons), but we can't always use one or the other (because we support JS-disabled browsers). Happy to go into this in more depth.