Accessibility: search button contains no text for readers

[K-T-O]

Pa11y files an error about the following issue:

• WCAG2AA.Principle4.Guideline4_1.4_1_2.H91.Button.Name

Error: This button element does not have a name available to an accessibility API. Valid names are: title attribute, element content.

This relates to the search button functionality of the "gov-search" component.

[fabiotisci]

@scarlz will look into this ;)