Add show_date_span parameter to channel calendar

[imacrayon]

Overview

Added a show_date_span="yes" parameter to the {exp:channel:calendar} tag which repeats the entry across every calendar "day" cell between the entry’s publish and expiration date.

Given a single entry (My Event) with an entry_date of April 1, 2019 and an expiration_date of April 5, 2019 - setting show_date_span="yes" would display a calendar with the first week like this:

S	M	T	W	T	F	S
31	1 My Event	2 My Event	3 My Event	4 My Event	5 My Event	6

Entires without an expiration_date will continue to show on a single day.

When combined with show_future_entries="yes", this would be useful for anyone wanting to build a simple event calendar.

Nature of This Change

• 🐛 Fixes a bug
• 🚀 Implements a new feature
• 🛁 Refactors existing code
• 💅 Fixes coding style
• ✅ Adds tests
• 👽 Adds new dependency
• 🔥 Removes unused files / code
• 🔒 Improves security

Is this backwards compatible?

• Yes
• No

Documentation

ExpressionEngine/ExpressionEngine-User-Guide#41

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[intoeetive]

@imacrayon thank you for this contribution!

Since EE5 is only receiving security updates at this point, would you be willing to update this PR for EE6, make it to be against 6.dev and also sign our updated contributor license agreement?