improve runner extend ui on slack

[umezawatakeshi]

part of https://github.com/cybozu-go/meows/issues/22

• Slack message now have two buttons - "Extend 2 hours" and "Delete immediately"
• Extention is limited to "6 hours from now"

Signed-off-by: UMEZAWA Takeshi takeshi-umezawa@cybozu.co.jp