Replace existing autoloader with Composer-supplied

- Move Friendica namespace to `src`
- Move required `ezyang/htmlpurifier` to vendor
- Remove existing static autoloader
- Change boot.php reference to the autoloader

vendor/ezyang/htmlpurifier/library/HTMLPurifier/ConfigSchema/schema/HTML.Allowed.txt

@@ -0,0 +1,25 @@
+HTML.Allowed
+TYPE: itext/null
+VERSION: 2.0.0
+DEFAULT: NULL
+--DESCRIPTION--
+
+<p>
+    This is a preferred convenience directive that combines
+    %HTML.AllowedElements and %HTML.AllowedAttributes.
+    Specify elements and attributes that are allowed using:
+    <code>element1[attr1|attr2],element2...</code>.  For example,
+    if you would like to only allow paragraphs and links, specify
+    <code>a[href],p</code>.  You can specify attributes that apply
+    to all elements using an asterisk, e.g. <code>*[lang]</code>.
+    You can also use newlines instead of commas to separate elements.
+</p>
+<p>
+    <strong>Warning</strong>:
+    All of the constraints on the component directives are still enforced.
+    The syntax is a <em>subset</em> of TinyMCE's <code>valid_elements</code>
+    whitelist: directly copy-pasting it here will probably result in
+    broken whitelists. If %HTML.AllowedElements or %HTML.AllowedAttributes
+    are set, this directive has no effect.
+</p>
+--# vim: et sw=4 sts=4