Use Remex in Sanitizer::stripAllTags()

Use Remex in Sanitizer::stripAllTags()

Using a real HTML tokenizer fixes bugs when < or > appear in attribute
values. The old implementation used delimiterReplace(), which didn't
handle this case:

    > print Sanitizer::stripAllTags( '<p data-foo="a&lt;b>c">Hello</p>' );
    c">Hello

We also can't use PHP's built-in strip_tags() because it doesn't handle
<?php and <? correctly:

    > print strip_tags('1<span class="<?php">2</span>3');
    1
    > print strip_tags('1<span class="<?">2</span>3');
    1

Bug: T179978
Change-Id: I53b98e6c877c00c03ff110914168b398559c9c3e