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.TidyLevel.txt

@@ -0,0 +1,24 @@
+HTML.TidyLevel
+TYPE: string
+VERSION: 2.0.0
+DEFAULT: 'medium'
+--DESCRIPTION--
+
+<p>General level of cleanliness the Tidy module should enforce.
+There are four allowed values:</p>
+<dl>
+    <dt>none</dt>
+    <dd>No extra tidying should be done</dd>
+    <dt>light</dt>
+    <dd>Only fix elements that would be discarded otherwise due to
+    lack of support in doctype</dd>
+    <dt>medium</dt>
+    <dd>Enforce best practices</dd>
+    <dt>heavy</dt>
+    <dd>Transform all deprecated elements and attributes to standards
+    compliant equivalents</dd>
+</dl>
+
+--ALLOWED--
+'none', 'light', 'medium', 'heavy'
+--# vim: et sw=4 sts=4