Introduce WOPI endpoints for local file access

[kendy]

This is an Easy Hack.
Potential mentors: @kendy

When the Collabora Online is built with --enable-debug, it is possible to open local files for easy testing. The problem with the current implementation is that it is a separate code path from what is later used in the real world - which always integrates through WOPI.

This task has two parts:

1. Introduce serving the local files via WOPI endpoints directly from loolwsd.
2. Kill the implementation of the local file storage, and do appropriate changes in the loolwsd.xml setting.

Ad 1) - this can be done in wsd/FileServer.cpp, see the FileServerRequestHandler::handleRequest - which would implement the GET and POST methods appropriately. For an inspiration what endpoints have to be implemented, have a look at test/WopiTestServer.hpp, method handleHttpRequest() that does the same thing for unit tests.

Ad 2) - git grep LocalStorage and kill all the uses. Then update the message that outputs the URL to be issued locally with the right one (git grep LOOLWSD_TEST_DOCUMENT_RELATIVE_PATH) and also check the setting in loolwsd.xml.in (the filesystem option).

[pedropintosilva]

fixed by #3226