Fix PHP Notices: Reduce error log noise

While DokuWiki suppresses PHP Notices they are still a code smell and
should be fixed. This fixes some PHP Notices that occurred.

Some of these fixes could b

Fix PHP Notices: Reduce error log noise

While DokuWiki suppresses PHP Notices they are still a code smell and
should be fixed. This fixes some PHP Notices that occurred.

Some of these fixes could be refactored into nicer code once we move to
PHP 7 and get access to the `??` operator.

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Create valid empty options

The HTML-spec[0] specifies:

> If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.

So a non-breakin

Create valid empty options

The HTML-spec[0] specifies:

> If the element has no label attribute and is not a child of a datalist element: Text that is not inter-element whitespace.

So a non-breaking space is valid solution.

[0] https://html.spec.whatwg.org/multipage/form-elements.html#the-option-element

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allow tests against ajax requests

cleaning after better understanding of the surroundings

back slashes in @import

cleaning

fix tests on windows

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doc fix

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using $options->getCmd to retrieve show-pages parameter

added option to wantedpages.php to show or not show pages where broken links occur

show pages where broken links occur: page_id => broken_link

clean extension names. fixes #2042

This makes sure the provided extension ID is a valid id before
initializing it in the extension manager.

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simplify ajax call handling

No need to check those seperately. The correct order should be ensured
already.

filter special chars from ajax call parameter. fixes #2019

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Update html.php

$regex initialization added to clearify that it is passed by reference and filled from subroutines

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