fix(VCalendar): fixed day width in month view on Firefox

[igadmile]

Description

As per specification, flex child elements should have min-width:0 property (min-height for columns) set
to prevent element overflow. See https://bugzilla.mozilla.org/show_bug.cgi?id=1114904

Motivation and Context

fix #9058

How Has This Been Tested?

Visually

Markup:

<template>
  <v-sheet height="600" width="300">
    <v-calendar
      v-model="focus"
      color="primary"
      :events="events"
      :event-margin-bottom="3"
      :now="today"
      type="month"
    ></v-calendar>
  </v-sheet>
</template>

<script>
export default {
  data: () => ({
    today: "2019-01-08",
    focus: "2019-01-08",
    events: [
      {
        name: "Very looong",
        start: "2019-01-14 18:00",
        end: "2019-01-14 19:00",
        color: "#4285F4"
      }
    ]
  })
};
</script>

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and breaking changes).
• My code follows the code style of this project.
• I've added relevant changes to the documentation (applies to new features and breaking changes in core library)
• I've added new examples to the kitchen (applies to new features and breaking changes in core library)

[dsseng]

reference FF bug too

[dsseng]

LGTM

[dsseng]

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