WIP: Add guests and comments to VEVENT

[miaulalala]

Fixes #23927

[github-advanced-security]

Found 19 potential problems in the proposed changes. Check the Files changed tab for more details.

[miaulalala]

We possibly only need to add the GUEST propety as COMMENT is an RFC defined field and should ideally be supported by the scheduling plugin (i. e. fix it upstream).

[miaulalala]

Relevant quote from the RFC:

3.8.1.4.  Comment

   Property Name:  COMMENT

   Purpose:  This property specifies non-processing information intended
      to provide a comment to the calendar user.

   Value Type:  TEXT

   Property Parameters:  IANA, non-standard, alternate text
      representation, and language property parameters can be specified
      on this property.

   Conformance:  This property can be specified multiple times in
      "VEVENT", "VTODO", "VJOURNAL", and "VFREEBUSY" calendar components
      as well as in the "STANDARD" and "DAYLIGHT" sub-components.

   Description:  This property is used to specify a comment to the
      calendar user.

   Format Definition:  This property is defined by the following
      notation:

       comment    = "COMMENT" commparam ":" text CRLF

       commparam  = *(
                  ;
                  ; The following are OPTIONAL,
                  ; but MUST NOT occur more than once.
                  ;
                  (";" altrepparam) / (";" languageparam) /
                  ;
                  ; The following is OPTIONAL,
                  ; and MAY occur more than once.
                  ;
                  (";" other-param)
                  ;
                  )

   Example:  The following is an example of this property:

       COMMENT:The meeting really needs to include both ourselves
         and the customer. We can't hold this meeting without them.
         As a matter of fact\, the venue for the meeting ought to be at
         their site. - - John

[miaulalala]

Won't fix for now :/