User migration

[jotoeri]

Implementing the user migration for NC>24 :)

• Submission loading shifted from Api-Controller to Service for reusability
• Forms are exported including questions, and submissions.
• Shares are not exported, as they will anyways change on a new instance.
• UserIds on Submissions are mapped to federation UIDs to still have a connection on who submitted, but to avoid accidential wrong mapping on new instance.

Implements #582

[codecov]

Codecov Report

Merging #1243 (1f0d906) into master (0d042b3) will increase coverage by 4.30%.
The diff coverage is 81.61%.

❗ Current head 1f0d906 differs from pull request most recent head 9713694. Consider uploading reports for the commit 9713694 to get more accurate results

@@             Coverage Diff              @@
##             master    #1243      +/-   ##
============================================
+ Coverage     32.60%   36.91%   +4.30%     
- Complexity      479      506      +27     
============================================
  Files            46       47       +1     
  Lines          1843     1956     +113     
============================================
+ Hits            601      722     +121     
+ Misses         1242     1234       -8     

[jotoeri]

Hmm, tests are of course only successful on NC24+. @nickvergessen can i somehow block these tests if NC<24?

[nickvergessen]

if (\OC_Util::getVersion()[0] < 24) {
$this->markTestSkipped('Not available in 22+23');
}

[come-nc]

Nice implementation. I did not test it yet.