[BUGFIX] Styling of security warnings

* List style is missing.
* Get rid of inline styles.
* Render p instead of li if only one message comes up (e.g. if reports module is installed).

Change-Id: Ic24b92c74eb55cb54cc3f76d2207c3b10ab4cbea
Releases: 4.6, 4.5
Fixes: #28181
Reviewed-on: http://review.typo3.org/3314
Reviewed-by: Georg Ringer
Tested-by: Georg Ringer
Reviewed-by: Stefan Neufeind
Reviewed-by: Susanne Moog
Tested-by: Susanne Moog