Allow shared avatars only for multisite

[ocean90]

Description of the Change

Fixes #221.

Changes is_avatar_shared() to always return false if not used in a multisite. Brings it in line with

simple-local-avatars/includes/class-simple-local-avatars.php

Lines 580 to 594 in 801cc93

	if ( is_multisite() ) {
	add_settings_field(
	'simple-local-avatars-shared',
	__( 'Shared network avatars', 'simple-local-avatars' ),
	array( $this, 'avatar_settings_field' ),
	'discussion',
	'avatars',
	array(
	'class' => 'simple-local-avatars',
	'key' => 'shared',
	'desc' => __( 'Uploaded avatars will be shared across the entire network, instead of being unique per site', 'simple-local-avatars' ),
	'default' => 1,
	)
	);
	}

How to test the Change

See #221

Changelog Entry

Fixed - Prevent PHP fatal error when switching from a multisite to single site installation.

Credits

Props @ocean90

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.