Make "First day of the week" configurable globally

Description

Currently, our datepickers require the consumers to set a first day of the week each time they are used:

<gl-datepicker first-dat="1" ...>...

This leads to inconsistencies in the localization of our UI, becuase often, we forget to explicitly set the first day, issues:

• Child Items do not respect "First day of the we... (gitlab#518649 - closed)
• Access-token date picker does not respect first... (gitlab#364327 - closed)
• Time tracking date picker does not respect star... (gitlab#442299 - closed)
• Time tracking calendar does not take "First day... (gitlab#422500 - closed)
• Work item calendars do not recognize start day ... (gitlab#525611 - closed)

Proposal

Extend the configuration mechanism at src/config.js to include locale-specific configuration, like the "first day of the week".

For example, our gitlab configuration could look like:

applyGitLabUIConfig({
  translations: ...
  firstDayOfWeek: gon.first_day_of_week
})