Convert system and themes to PSR-4

[craigh]

Convert all system modules and all system themes to PSR-4. maintain BC.

REQUIRES COMPOSER UPDATE !!!

Q	A
Bug fix?	no
New feature?	yes
BC breaks?	no
Deprecations?	yes*
Tests pass?	yes
Fixed tickets	-
Refs tickets	#2424
License	MIT
Doc PR	-
Changelog updated	yes

• deprecated items are only a result of namespace changes and a BC layer has been added to compensate.

[craigh]

I think this is ready but would really appreciate some additional testing. @Guite @phaidon @cmfcmf @Kaik

[craigh]

refs #2424

[Kaik]

I was away for some time now I hope to be able to come back to this soon I need few more days and I will do all the tests and check all the new stuff.

[craigh]

I think this is ready but would really appreciate some additional testing.

wow, was I wrong. 🎱 ⛵

but now, I think it really is ready, but nobody tests with me. 😢 😭

[craigh]

Will be merging this Sunday afternoon (EDT) if there are no objections. @Guite @phaidon @cmfcmf @Kaik

[Guite]

I just tested a new installation on my local machine which worked fine. Afterwards I clicked a bit through administration and changed several settings. This worked flawlessly as well.

So 👍 from me - and thank your for all this work!

[cmfcmf]

Thank you @craigh.