[FEATURE] Install tool: Left menu at fixed position

* Streamlines install tool main template.
* Adds a ContextService and use that in the templates.
  No more huge if/then/else constructs to differentiate
  standalone and backend modes.
* Fixes the left menu (and TYPO3 logo), letting the
  rest scroll up and down.
* In backend mode the menu and content starts right on top
  (no further headers to make scrolling look good)

Releases: 6.2
Resolves: #50910
Change-Id: Ie2ec277026a82332b533af51a37e7f308dd24e8c
Reviewed-on: https://review.typo3.org/23798
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny