Feature/select multiple

[haroun]

Summary

Add multiple and size option to select field widget

What are the specific steps to test this change?

1. Add a select field widget to a form
2. In the widget editor modal, set Allow multiple options to be selected to Yes
3. Set the size to the value you want.
4. Check the rendered template in Apostrophe admin UI and on the website

What kind of change does this PR introduce?

(Check at least one)

• Bug fix
• New feature
• Refactor
• Documentation
• Build-related changes
• Other

Make sure the PR fulfills these requirements:

• It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• The changelog is updated
• Related documentation has been updated
• Related tests have been updated

If adding a new feature without an already open issue, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

[boutell]

Looks good, but please put it behind a module-level option that defaults to false. The user experience of "out of the box" HTML multiple select is not good and we don't have time to pursue designing a better experience and doing progressive enhancement in this cycle, per Alex. I know that for the project you have in mind this is less of an issue because the UI is rebuilt in Vue anyway.