fix(GlDatepicker): use defaultDate as an initial value

What does this MR do?

• When defaultDate prop is used, set setDefaultDate option as true.
• Adds documentation on defaultDate

Background

Using value prop of GlDatepicker to set the initial date has a serious side-effect. Because value is a watched property, setDate is always triggered emitting input event. Using value was one of the major causes of the bug reported in gitlab#329614 (closed).

However, when using defaultDate, the initial date isn't set in the pickaday calendar. This MR fixes that.

❗ According to the pickaday README, defaultDate should be used to set the initial date.

defaultDate the initial date to view when first opened

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • CustomersDot: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• Security reports checked/validated by a reviewer from the AppSec team

Accessibility

If this MR adds or modifies a component, take a few moments to review the following:

• All actions and functionality can be done with a keyboard.
• Links, buttons, and controls have a visible focus state.
• All content is presented in text or with a text equivalent. For example, alt text for SVG, or aria-label for icons that have meaning or perform actions.
• Changes in a component’s state are announced by a screen reader. For example, changing aria-expanded="false" to aria-expanded="true" when an accordion is expanded.
• Color combinations have sufficient contrast.