PoC: SystemTags endpoint to return tags used by a user with meta data

PoC: SystemTags endpoint to return tags used by a user with meta data

Target case is photos app: when visiting the tags category, all systemtags
of the whole cloud are retrieved. In subequent steps the next tag is
requested until the browser view is filled with tag tiles (i.e. previews
are requested just as well).

With this approach, we incorpoate the dav search and look for user related
tags that are used by them, and already returns the statistics (number of
files tagged with the respective tag) as well as a file id for the purpose
to load the preview. This defaults to the file with the highest id.

Call:
curl -s -u 'user:password' \
  'https://my.nc.srv/remote.php/dav/systemtags-current' \
  -X PROPFIND -H 'Accept: text/plain' \
  -H 'Accept-Language: en-US,en;q=0.5'  -H 'Depth: 1' \
  -H 'Content-Type: text/plain;charset=UTF-8' \
  --data @/home/doe/request-systemtag-props.xml

With request-systemtag-props.xml:
<?xml version="1.0" encoding="UTF-8"?>
<d:propfind xmlns:d="DAV:">
        <d:prop xmlns:oc="http://owncloud.org/ns" xmlns:nc="http://nextcloud.org/ns">
                <oc:id/>
                <oc:display-name/>
                <oc:user-visible/>
                <oc:user-assignable/>
                <oc:can-assign/>
                <nc:files-assigned/>
                <nc:reference-fileid/>
        </d:prop>
</d:propfind>

Example output:
  …
  <d:response>
    <d:href>/master/remote.php/dav/systemtags/84</d:href>
    <d:propstat>
      <d:prop>
        <oc:id>84</oc:id>
        <oc:display-name>Computer</oc:display-name>
        <oc:user-visible>true</oc:user-visible>
        <oc:user-assignable>true</oc:user-assignable>
        <oc:can-assign>true</oc:can-assign>
        <nc:files-assigned>42</nc:files-assigned>
        <nc:reference-fileid>924022</nc:reference-fileid>
      </d:prop>
      <d:status>HTTP/1.1 200 OK</d:status>
    </d:propstat>
  </d:response>
  <d:response>
    <d:href>/remote.php/dav/systemtags/97</d:href>
    <d:propstat>
      <d:prop>
        <oc:id>97</oc:id>
        <oc:display-name>Bear</oc:display-name>
        <oc:user-visible>true</oc:user-visible>
        <oc:user-assignable>true</oc:user-assignable>
        <oc:can-assign>true</oc:can-assign>
        <nc:files-assigned>1</nc:files-assigned>
        <nc:reference-fileid>923422</nc:reference-fileid>
      </d:prop>
      <d:status>HTTP/1.1 200 OK</d:status>
    </d:propstat>
  </d:response>
  …

Signed-off-by: Arthur Schiwon <blizzz@arthur-schiwon.de>