Add option to render enums as select dropdown

[gjvoosten]

When so defined in the dictionary, an enum will be a single select, an enumset will be a multiselect. The number of visible entries in the select can be set with the htmlSize property.

Closes AB#994

User changes

• None

Superuser changes

• None

Admin changes

• None

System admin changes

• anet.yml or anet-dictionary.yml needs change
  You can now do:

        someField:
          type: enum
          asA: select
  

  to render an enum as a dropdown instead of a row of buttons, or:

        someField:
          type: enumset
          asA: select
          htmlSize: 4
  

  to render an enumset as a multiselect dropdown, with 4 visible choices.
• db needs migration
• documentation has changed
• graphql schema has changed

Checklist

• Described the user behavior in PR body
• Referenced/updated all related issues
• commits follow a repo#issue: Title title format and these 7 rules
• commits have a clean history, otherwise PR may be squash-merged
• Added and/or updated unit tests
• Added and/or updated e2e tests
• Added and/or updated data migrations
• Updated documentation
• Resolved all build errors and warnings
• Opened debt issues for anything not resolved here