Unify accepted templates

[elzody]

There are several places in the code that define what file types are accepted as templates (see #5475 and #5512). Instead of trying to make them all match, we should refactor it a bit and define them once and use that where needed.

richdocuments/lib/Service/InitialStateService.php

Lines 73 to 79 in e14e886

	'acceptedTemplateTypes' => [
	'.ott', '.otg', '.otp', '.ots',
	'.odt', '.odg', '.odp', '.ods',
	'.dot', '.dotx', '.doc', '.docx',
	'.xlt', '.xltx', '.xls', '.xlsx',
	'.pot', '.potx', '.ppt', '.pptx',
	],

richdocuments/lib/TemplateManager.php

Lines 507 to 513 in 6033f3a

	$supportedExtensions = [
	'ott', 'otg', 'otp', 'ots',
	'odt', 'odg', 'odp', 'ods',
	'dotx', 'xltx', 'potx',
	'docx', 'xlsx', 'pptx',
	'dot', 'xlt', 'pot',
	];