Change to using inbuilt shiny select components

[cjrace]

Currently we use select components from shinyGovstyle. Though these are plain HTML behind the scenes and don't have additional features, for example - optimisations for larger lists.

We should instead use the selectInput / selectizeInput that comes with shiny and apply styling to ensure accessibility. Starters for this can be seen in the apprenticeships dashboard.

Key issues we should check:

• Correct labelling markup
• Focus indicators
• Fonts

I'm assuming once we've worked out the main examples, we should then build in our own function into dfeshiny.

In the app we should provide two examples, one of the basic select component, and one as a combobox / multi-select. The latter is likely to have accessibility issues, though that makes it even more important to get into the template so that we can test it.